Gear oil is one of the most precisely specified lubricants in Australian industry — and one of the most frequently substituted with the wrong product. A maintenance fitter who reaches for hydraulic oil because it's on the shelf and "looks about the same viscosity" can destroy a gearbox in a matter of weeks without leaving any obvious sign of what happened. The gear tooth scuffing that results gets attributed to overloading or misalignment rather than the real cause.
This guide covers everything an industrial maintenance fitter needs to select the right gear oil: the three grading systems you'll encounter on gearbox nameplates (ISO VG, AGMA, and SAE), what the GL rating actually means and when it matters, how to select gear oil by gearbox type, and the special case of worm gear oil that catches out even experienced tradespeople. For a broader overview of all industrial lubricant types, see the AIMS Industrial Lubricants Guide.
How Gear Oil Is Graded: ISO VG, AGMA, and SAE Explained — Quick Reference
Gear oil viscosity is communicated in three different systems depending on whether the gearbox is industrial or automotive. You will encounter all three on Australian worksites.
| AGMA Grade | Equivalent ISO VG | Typical Application |
|---|---|---|
| AGMA 2 | ISO VG 100 | Lightly loaded, high-speed enclosed gear drives |
| AGMA 3 | ISO VG 150 | General purpose, moderate speed enclosed drives |
| AGMA 4 | ISO VG 220 | Most common industrial gearbox grade (helical, bevel) |
| AGMA 5 | ISO VG 320 | Heavier loads, lower speed industrial gearboxes |
| AGMA 6 | ISO VG 460 | Heavy duty, low speed enclosed drives |
| AGMA 7 | ISO VG 680 | Very heavy duty, very low speed or high load |
| AGMA 8 | ISO VG 1000 | Extreme duty, open gear applications |
| AGMA 8A Comp | ISO VG 1000 compounded | Open worm gears (with fatty oil additive) |
What Is Gear Oil — and How It Differs from Other Industrial Oils
Gear oil is a lubricant formulated specifically for the meshing contact between gear teeth. The defining characteristic that sets it apart from every other industrial oil is its extreme pressure (EP) additive package. When two gear teeth mesh, the contact pressure at the tooth face can exceed 1,000 MPa — far beyond what ordinary oil films can withstand. Without EP additives, the oil film collapses at the contact point, metal welds to metal, and the weld shears as the tooth passes through. Repeat this millions of times and you get scuffing, pitting, and eventual tooth failure.
EP additives — typically sulphur-phosphorus compounds or borate chemistry — solve this by activating under high contact temperature and pressure to form a thin, sacrificial iron sulphide film on the gear tooth surface. This film shears preferentially, protecting the underlying metal. It is consumed in service, which is why gear oil needs periodic replacement.
The critical difference from other industrial oils:
- Hydraulic oil contains anti-wear (AW) additives, not EP additives. AW additives protect pump internals under moderate pressure; they provide no protection against gear tooth contact. Using hydraulic oil in a gearbox guarantees tooth scuffing under load.
- Compressor oil is formulated for low oxidation residue at compression temperatures. It has neither AW nor EP protection for gear teeth.
- Way oil contains anti-stick-slip (tackiness) additives. Correct for machine tool slideways; unsuitable for gearboxes.
- Engine oil contains detergent additives that can cause foaming in gearboxes and may not provide adequate EP performance — despite having similar viscosity numbers to some gear oil grades (see the viscosity section below).
The lubricants are not interchangeable. The ISO VG number tells you the viscosity; it says nothing about the additive package. An ISO VG 68 hydraulic oil and an ISO VG 68 gear oil are entirely different products. For hydraulic system selection, see the AIMS Hydraulic Oil Guide.
How Gear Oil Is Graded: ISO VG, AGMA, and SAE Explained
Gear oil viscosity is communicated in three different systems depending on whether the gearbox is industrial or automotive. You will encounter all three on Australian worksites. Understanding which system a gearbox nameplate is using — and how to convert between them — is essential for correct oil selection.
ISO VG (Viscosity Grade)
The ISO viscosity grade system defines kinematic viscosity in centistokes (cSt) at 40°C. ISO VG 100 means the oil has a viscosity of approximately 100 cSt at 40°C, within a ±10% tolerance band. Industrial gearboxes typically operate in the ISO VG 68 to ISO VG 680 range, depending on speed and load. Higher ISO VG grades are thicker (more viscous) and are used in slower, higher-load applications. Lower grades are used in faster, lighter applications.
AGMA (American Gear Manufacturers Association)
The AGMA classification system is used in Australian industrial gearbox specifications from manufacturers such as Dodge, Rexnord, Baldor, and many OEM nameplate gearboxes on industrial equipment. AGMA grades are numerical values that correspond to ISO VG ranges:
| AGMA Grade | Equivalent ISO VG | Typical Application |
|---|---|---|
| AGMA 2 | ISO VG 100 | Lightly loaded, high-speed enclosed gear drives |
| AGMA 3 | ISO VG 150 | General purpose, moderate speed enclosed drives |
| AGMA 4 | ISO VG 220 | Most common industrial gearbox grade (helical, bevel) |
| AGMA 5 | ISO VG 320 | Heavier loads, lower speed industrial gearboxes |
| AGMA 6 | ISO VG 460 | Heavy duty, low speed enclosed drives |
| AGMA 7 | ISO VG 680 | Very heavy duty, very low speed or high load |
| AGMA 8 | ISO VG 1000 | Extreme duty, open gear applications |
| AGMA 8A Comp | ISO VG 1000 compounded | Open worm gears (with fatty oil additive) |
If your gearbox nameplate specifies "AGMA 4 EP", you need an ISO VG 220 gear oil with EP additives. If it specifies "AGMA 5 Comp", you need a compounded gear oil at ISO VG 320 — not a standard EP gear oil.
For the full AGMA-to-ISO VG cross-reference across all grades — plus SAE gear oil equivalents — see the ISO VG viscosity chart.
SAE (Society of Automotive Engineers)
SAE gear oil grades — such as SAE 75W-90, SAE 80W-90, and SAE 85W-140 — are used for automotive differential and manual gearbox applications and are defined under SAE J306. These grades describe performance across a temperature range: the "W" number describes cold-temperature viscosity (W = winter), and the second number describes high-temperature viscosity.
Important: SAE gear oil viscosity numbers are entirely different from SAE engine oil viscosity numbers. This is one of the most common sources of confusion in Australian workshops. SAE 90 gear oil is approximately the same viscosity as SAE 50 engine oil — far heavier than any SAE 90 engine oil. The two numbering systems use different viscosity test methods and cannot be compared directly.
| SAE Gear Grade | Approximate ISO VG Equivalent | Typical Application |
|---|---|---|
| SAE 75W-90 | ISO VG 100 | Manual gearboxes, transaxles, light differentials |
| SAE 80W-90 | ISO VG 100–150 | General purpose differentials and manual gearboxes |
| SAE 85W-140 | ISO VG 220 | Heavy duty differentials, hypoid axles, high-load applications |
SAE gear grades are primarily used for automotive and fleet maintenance applications. For industrial gearboxes — conveyor drives, mixer gearboxes, helical reducers, worm drives — always use the ISO VG or AGMA grade specified on the nameplate. If the nameplate is absent or unreadable, contact the gearbox manufacturer for the specification.
EP Additives: What the GL Rating Actually Means
The API GL classification (GL stands for Gear Lubricant) defines the extreme pressure performance level of a gear oil. The GL number tells you how much EP protection the oil provides and for what type of gear contact it is suitable. Using the wrong GL grade — either too low or too high — causes failure.
| GL Rating | EP Level | Sulphur-Phosphorus Content | Typical Application | Yellow Metal Safe? |
|---|---|---|---|---|
| GL-1 | None | None | Lightly loaded spur and helical gears, manual transmissions with low loads | Yes |
| GL-2 | Minimal | Very low | Worm gears (historical — superseded by compounded oils) | Yes |
| GL-3 | Low EP | Low | Manual transmissions with moderate loads — limited use in modern applications | Yes |
| GL-4 | Moderate EP | Moderate (~0.5–2%) | Most industrial gearboxes, manual gearboxes, axles with helical or spiral bevel gears | Yes |
| GL-5 | High EP | High (~2–4%) | Hypoid gears (automotive differentials, severe duty axles) | ⚠️ No |
| GL-6 | Very high EP | Very high | High-offset hypoid gears — rare, largely superseded by GL-5 | No |
GL-4 vs GL-5: The Distinction That Matters Most
The choice between GL-4 and GL-5 is the most consequential gear oil specification decision for industrial maintenance fitters. Most industrial gearboxes specify GL-4. Most automotive differentials specify GL-5. Getting them mixed up causes equipment damage in both directions.
GL-4 contains a moderate level of sulphur-phosphorus EP additives — enough to protect helical, spur, and spiral bevel gear teeth from scuffing under typical industrial loads. It is safe for brass and bronze components, which appear in gearbox bearings, thrust washers, and in some older industrial gearbox housings.
GL-5 contains two to four times the sulphur-phosphorus concentration of GL-4. The higher EP performance protects hypoid gear sets — where the pinion centreline is offset below the ring gear centreline, creating a severe wiping contact rather than pure rolling. This is the standard geometry for automotive rear differentials. GL-5 is not inherently "better" than GL-4 — it is formulated for a different type of gear contact.
The problem with using GL-5 where GL-4 is specified: the elevated sulphur content in GL-5 reacts chemically with copper alloys — brass, bronze, and phosphor-bronze. This sulphur corrosion attacks:
- Synchromesh rings in manual gearboxes (most synchros are brass)
- Worm wheel faces (typically phosphor-bronze)
- Bronze thrust bearings and washers in industrial gearboxes
The corrosion is slow and progressive. A gearbox filled with GL-5 instead of GL-4 will not fail immediately — it will fail over 6–18 months of operation as the bronze components corrode. By the time failure occurs, the connection to the wrong oil grade has usually been lost. Always check the OEM specification before selecting between GL-4 and GL-5, and always err toward GL-4 if you are uncertain and the equipment contains any yellow metal components.
EP Additive Chemistry: Sulphur-Phosphorus vs Borate
Most EP gear oils use sulphur-phosphorus (S-P) chemistry. S-P additives are thermally activated — they react with the gear tooth surface only when contact temperature exceeds approximately 150–180°C, which occurs under heavy load. This makes them effective under the conditions they are designed for and relatively inert under lighter loads.
Borate EP additives (boron-based chemistry) work differently. They activate at lower temperatures and lower contact pressures, providing EP film even under moderate loads. Borate-based gear oils are used where consistent EP protection is required across a wider load range — including some food-safe and zinc-free formulations. Borate EP oils are generally safer for yellow metals than sulphur-phosphorus GL-5 oils. If a gearbox contains bronze components and requires EP performance beyond GL-4, a borate-based EP oil is worth investigating.
Gear Oil Selection by Gearbox Type
The gearbox type determines the contact geometry between gear teeth, which determines the level of EP protection needed. Four gearbox types cover the large majority of Australian industrial applications.
Spur and Helical Gearboxes
Spur and helical gears are the most common in Australian industrial facilities — conveyor drives, mixer gearboxes, pump drives, fan drives, and general power transmission. The tooth contact is primarily rolling with a small sliding component. EP requirements are moderate.
- ISO VG grade: ISO VG 220 for most industrial applications running at normal ambient temperatures (15–35°C). ISO VG 150 for higher-speed drives or cooler climates. ISO VG 320 for slow-speed, heavily loaded drives or hot environments (>40°C ambient).
- GL rating: GL-4 is adequate for most spur and helical drives. The OEM specification prevails — check the nameplate or service manual.
- Key mistake: Selecting ISO VG 320 or 460 for a standard helical drive because "thicker is better." Over-viscous gear oil increases churning losses, raises operating temperature, and may reduce oil penetration to critical surfaces. Follow the OEM specification.
Bevel Gearboxes
Bevel gearboxes transmit power between intersecting shafts (typically at 90°). Spiral bevel gears have a higher sliding contact component than spur or helical gears, requiring higher EP performance. Straight bevel gears have lower sliding contact and may be adequately served by GL-4.
- ISO VG grade: ISO VG 220 is most common. High-load applications may use ISO VG 320.
- GL rating: GL-4 to GL-5 depending on OEM specification and contact type. Check for yellow metal components before using GL-5.
Hypoid Gearboxes (Automotive Differentials)
Hypoid gears have a pronounced offset between the pinion and ring gear centrelines, creating a sliding-wiping contact that generates extreme contact temperatures. This is the geometry used in virtually all automotive rear differentials manufactured in Australia and by Australian-market vehicle brands. Standard EP performance (GL-4) is insufficient — GL-5 is required.
- SAE grade: SAE 80W-90 GL-5 for most light commercial and passenger vehicle differentials. SAE 85W-140 GL-5 for heavy commercial vehicles, towing applications, or hot operating environments.
- SAE 75W-90 GL-5: Used in some modern differentials where the OEM specifies synthetic gear oil for improved cold-weather performance or fuel economy. Always check the vehicle specification — do not substitute 80W-90 for 75W-90 in a specification that calls for the lighter grade.
- Limited-slip differentials: Require gear oil with friction modifier (LSD) additive. Standard GL-5 without the friction modifier will cause LSD chatter. Check for "LS" or "limited-slip" designation on the oil specification.
Worm Gearboxes — The Special Case
Worm gearboxes require different lubrication from any other gear type. This is covered in full in the next section.
Worm Gear Oil: The One That Breaks the Rules
Worm gearboxes are found throughout Australian industrial facilities — conveyor drives, mixer drives, packaging machinery, and any application requiring a high reduction ratio in a compact housing. They are identified by their characteristic 90° output shaft angle and high reduction capability (typically 5:1 to 100:1 in a single stage).
The geometry of a worm gearbox is fundamentally different from any other gear type. The steel worm (the driving member) engages a phosphor-bronze worm wheel in a sliding contact that sweeps across the tooth face rather than rolling through it. This contact generates high heat and requires a lubricant that can withstand continuous sliding without scoring the bronze tooth face.
The problem: sulphur-phosphorus EP additives — the chemistry in standard GL-4 and GL-5 gear oils — react chemically with phosphor-bronze. The sulphur attacks the copper in the bronze alloy, forming copper sulphide corrosion products that erode the worm wheel tooth surface progressively. A standard EP gear oil in a worm gearbox will corrode the bronze worm wheel and lead to premature failure — exactly the opposite of the protection it provides in other gearbox types.
What worm gearboxes need instead:
Option 1: Compounded Gear Oil
Compounded gear oil (designated "Comp" in AGMA specifications, such as AGMA 7 Comp or AGMA 8 Comp) is a mineral gear oil containing 3–10% fatty acid or synthetic fatty acid ester. The fatty component provides the anti-scoring lubrication that protects the bronze worm wheel without the corrosive sulphur chemistry of EP additives. Compounded gear oil is the standard choice for most industrial worm gearboxes in Australian facilities.
- ISO VG: ISO VG 220 Comp for most worm gearboxes. ISO VG 320 Comp or ISO VG 460 Comp for slow-speed, high-reduction, or heavily loaded worm drives. ISO VG 680 Comp for very slow-speed open worm gears.
- Availability: Compounded worm gear oil is stocked by most Australian industrial lubricant suppliers, including AIMS Industrial. Confirm the AGMA "Comp" designation — do not substitute standard EP gear oil at the same ISO VG grade.
Option 2: PAG Synthetic Gear Oil
Polyalkylene glycol (PAG) synthetic gear oil is the premium choice for worm gearboxes. PAG has several properties that make it superior to compounded mineral oil for worm drive lubrication:
- Film strength: PAG forms an intrinsically stronger oil film at the worm-wheel contact, providing better protection at higher loads and temperatures than mineral-based compounded oils.
- Efficiency: A worm gearbox running on PAG synthetic typically operates 3–8°C cooler than the same gearbox on mineral compounded oil, with a corresponding improvement in operating efficiency of 1–3%. Over a year of continuous operation, this efficiency gain can offset the cost premium of the synthetic oil.
- Extended drain interval: PAG in worm gearboxes typically runs 4,000–8,000 hours between changes versus 2,000–4,000 hours for mineral compounded oil.
- No yellow metal corrosion: PAG contains no sulphur chemistry and does not attack phosphor-bronze.
Critical PAG compatibility warning: PAG is not compatible with mineral gear oil, most standard oil seals (particularly nitrile rubber), and many paints. Before converting a worm gearbox from mineral compounded oil to PAG synthetic:
- Confirm the gearbox seals are PAG-compatible (polyacrylate or PTFE seals are; nitrile are not — check with the gearbox manufacturer)
- Flush the gearbox thoroughly to remove all mineral oil residue
- Never mix PAG with mineral oil — even small amounts of mineral oil contamination degrade PAG performance
For worm gearbox selection, sizing, and specification details, see the AIMS Worm Gearbox Selection Guide. For chain lubrication, see the AIMS Chain Lube Guide.
Mineral vs Synthetic Gear Oil: When the Upgrade Is Worth It
Mineral gear oil is the baseline product — refined petroleum base stocks with EP, anti-oxidant, and anti-foam additive packages. It is adequate for the large majority of industrial gearbox applications within normal operating temperature and speed parameters. Synthetic gear oil costs more per litre but offers measurable performance and cost benefits in specific situations.
Synthetic PAO (Polyalphaolefin)
PAO is the most common synthetic base for industrial gear oils and the most straightforward upgrade from mineral oil. PAO gear oil is a near drop-in replacement for mineral gear oil in most enclosed gearboxes — it uses the same EP additive systems and is compatible with standard seals and paints.
PAO advantages over mineral gear oil:
- Wider temperature range: PAO maintains adequate viscosity from –40°C to over 150°C operating temperature, versus approximately –15°C to 120°C for mineral gear oil. Relevant for gearboxes in cold-climate or high-temperature environments.
- Better viscosity stability: PAO viscosity index (VI) is typically 140–160 versus 90–100 for mineral oil. Higher VI means less viscosity change across the operating temperature range — the oil is less thin when hot and less thick when cold.
- Extended drain intervals: Typically 4,000–8,000 hours for PAO versus 2,000–4,000 for mineral oil. For gearboxes in hard-to-access locations or that require production shutdown for maintenance, extended drain intervals have real value.
- Lower pour point: PAO flows at temperatures as low as –40°C, which matters for outdoor equipment in southern Australian winters or refrigerated plant environments.
When PAO is worth the premium: continuous duty cycle (24/7 operation), high ambient temperature environments (northern Queensland, indoor furnace or oven areas), cold climate outdoor equipment, and gearboxes where oil change downtime is costly.
PAG (Polyalkylene Glycol)
PAG is the preferred synthetic for worm gearboxes (see above). For other gearbox types, PAG's compatibility constraints — non-compatible with mineral oil and many seals — make it a specialised choice rather than a general upgrade. PAG is also used in some food-safe gear oil formulations where the base oil must meet H1 (incidental food contact) requirements.
Synthetic Ester
Ester-based gear oils are biodegradable and used where environmental spill risk is a concern — forestry equipment, marine vessels, food processing facilities, and operations near waterways. Ester gear oils typically meet ISO VG 220 or 320 and carry EP additive packages. They are more expensive than PAO synthetics and are a compliance choice rather than a performance upgrade for most Australian industrial applications.
Gear Oil Change Intervals and Oil Analysis
Gear oil deteriorates through three main mechanisms: oxidation (thermally driven chemical degradation), contamination (water, dust, metallic wear debris), and additive depletion (EP and anti-oxidant additives consumed in service). Changing oil at the correct interval prevents all three from reaching the point of inadequate protection.
Typical Change Intervals
| Gearbox Type | Oil Type | Change Interval |
|---|---|---|
| Industrial enclosed gearbox | Mineral EP gear oil | 2,000–4,000 hours or annually |
| Industrial enclosed gearbox | Synthetic PAO | 4,000–8,000 hours or every 2 years |
| Worm gearbox | Mineral compounded | 2,000–4,000 hours or annually |
| Worm gearbox | PAG synthetic | 4,000–8,000 hours or every 2 years |
| Automotive differential | GL-5 mineral | 40,000–80,000 km or as per vehicle OEM |
| First fill (new gearbox) | Any | 500–1,000 hours (run-in interval) then resume normal schedule |
These are general guidelines. The OEM specification always takes priority. Gearboxes running in high-temperature environments, under sustained heavy loads, or in dusty or wet conditions should be on shorter change intervals. Change immediately after any event that risks water ingress — a seal failure, submersion, or extended operation with condensation present.
The first fill change interval is important and often skipped. New gearboxes generate fine metallic swarf during the run-in period as gear tooth surfaces bed together. This debris contaminates the initial oil charge and, if not removed, acts as an abrasive in subsequent service. A first change at 500–1,000 hours is insurance against premature wear.
Oil Analysis: When to Use It
Time-based oil changes are conservative by nature — they are set to protect the worst-case gearbox operating under the worst-case conditions. For gearboxes in lighter service, they result in oil being changed before it needs to be. For gearboxes in unexpectedly severe service, they may not be frequent enough.
Oil analysis closes this gap. A sample sent to a testing laboratory (cost: approximately $30–80 per sample in Australia) returns results within a few days covering:
- Viscosity: Confirms the oil has not significantly thinned (dilution) or thickened (oxidation/contamination)
- TAN (Total Acid Number): Tracks oxidation — rising TAN indicates additive depletion and oil degradation
- Water content: Detects seal failure and condensation — water in gear oil forms emulsions that strip the oil film and promote corrosion
- ICP spectrometry (wear metals): Iron (gear teeth and housing), copper (bronze components, bearings), lead (bearing shells), and silicon (dust contamination) identify which components are wearing and at what rate
Oil analysis makes commercial sense for any gearbox where unplanned failure would cost more than several hundred dollars in lost production or replacement cost — which in practice means most industrial gearboxes. A programme that catches a failing gearbox seal or unusual iron levels before tooth failure occurs repays its cost many times over.
5 Common Gear Oil Mistakes
1. Using Hydraulic Oil in a Gearbox
The most common error, and the most damaging. Hydraulic oil contains AW (anti-wear) additives designed for pump internals under moderate pressure — not EP additives designed for gear tooth contact under extreme pressure. A gearbox filled with hydraulic oil will run for weeks or months before tooth scuffing becomes severe enough to cause failure. By that time the oil has usually been changed and the connection to the wrong product is lost. The failure is recorded as mechanical, not lubrication-related. Check: gear oil has EP additives; hydraulic oil does not. They are not interchangeable.
2. Using GL-5 Where GL-4 Is Specified
More EP additives is not always better. If the OEM specifies GL-4 and the gearbox contains brass or bronze components, using GL-5 introduces sulphur chemistry that will corrode those components over time. The corrosion is slow — it may take six to eighteen months to show as increased wear or rough operation. Always check OEM specification, always check for yellow metal components before substituting GL-5 for GL-4.
3. Confusing SAE Gear Oil and SAE Engine Oil Viscosity Numbers
SAE 90 gear oil is not the same viscosity as SAE 90 engine oil. SAE 90 gear oil is approximately equivalent to SAE 50 engine oil in terms of kinematic viscosity. The two systems use different test methods and different viscosity scales. If a gearbox nameplate calls for SAE 90 gear oil and you fill it with SAE 90 engine oil, you are using an oil that is far too thin — and one with the wrong additive package.
4. Substituting Standard EP Gear Oil in a Worm Gearbox
Worm gearboxes with bronze worm wheels cannot use standard sulphur-phosphorus EP gear oil. Many maintenance fitters are unaware of this and use whatever GL-4 gear oil is on the shelf at the correct ISO VG grade. The result is progressive bronze corrosion and eventual worm wheel failure. Worm gearboxes require compounded gear oil (AGMA Comp designation) or PAG synthetic. If unsure, look up the gearbox manufacturer's lubrication specification.
5. Ignoring Gearbox Temperature as a Diagnostic Signal
A gearbox that runs hotter than its rated operating temperature is telling you something is wrong — and reaching for a higher-viscosity oil is almost never the right response. Common causes of high gearbox temperature: incorrect oil fill level (both overfill and underfill cause temperature rise), incorrect oil grade, gear misalignment, overloading beyond rated capacity, blocked breather (pressure build-up), or deteriorated oil past its change interval. Measure and record gearbox operating temperature as part of routine maintenance. A sustained increase of more than 15°C above normal warrants investigation, not just an oil change.
Frequently Asked Questions
What is EP gear oil?
EP stands for extreme pressure. EP gear oil contains additives — typically sulphur-phosphorus compounds or borate chemistry — that activate under the high contact pressures between gear teeth, forming a thin sacrificial film that prevents metal-to-metal welding and scoring. Without EP protection, gear teeth scuff and fail under load. Standard hydraulic oil and engine oil do not contain EP additives and cannot substitute for gear oil in enclosed gearboxes.
What is the difference between GL-4 and GL-5 gear oil?
GL-4 and GL-5 are API classifications defining the extreme pressure performance level of a gear oil. GL-4 contains a moderate level of sulphur-phosphorus EP additives — suitable for most industrial gearboxes, manual vehicle gearboxes, and axles with helical or spiral bevel gears. GL-5 contains two to four times the sulphur-phosphorus concentration and is specified for hypoid gear sets (automotive differentials) where the contact geometry generates extreme heat. GL-5 is not a universal upgrade from GL-4 — its high sulphur content corrodes yellow metals (brass, bronze) found in many gearboxes that specify GL-4. Always follow the OEM specification.
Can I use GL-5 where GL-4 is specified?
No, not without first checking for yellow metal components. GL-5's elevated sulphur content corrodes brass and bronze — which appears in synchromesh rings, bronze thrust washers, and bronze worm wheels in many gearboxes. If the OEM specifies GL-4, substituting GL-5 will damage yellow metal components over time. The damage is progressive and slow — the connection to the wrong oil grade is often lost by the time failure occurs. Use GL-4 where GL-4 is specified.
What is the difference between ISO VG and SAE gear oil grades?
ISO VG grades (such as ISO VG 220) describe kinematic viscosity at 40°C in centistokes. They are used for industrial gearboxes. SAE gear grades (such as SAE 80W-90) describe viscosity performance across a temperature range and are used for automotive differentials and manual gearboxes. The two systems cannot be directly compared using the grade number alone. ISO VG 220 is approximately equivalent to SAE 85W-140. Additionally, SAE gear oil grades are entirely different from SAE engine oil grades — SAE 90 gear oil and SAE 90 engine oil are very different products with different viscosities.
What is AGMA gear oil?
AGMA (American Gear Manufacturers Association) grades are a classification system used in Australian industrial gearbox specifications to define gear oil viscosity. AGMA grades correspond to ISO VG ranges: AGMA 4 = ISO VG 220, AGMA 5 = ISO VG 320, AGMA 6 = ISO VG 460, and so on. If a gearbox nameplate specifies "AGMA 4 EP", use an ISO VG 220 gear oil with EP additives. If it specifies "AGMA 5 Comp", use an ISO VG 320 compounded gear oil — not a standard EP gear oil.
What gear oil does an industrial gearbox need?
Start with the gearbox nameplate or OEM service manual — it will specify the oil type (EP or compounded), ISO VG or AGMA grade, and change interval. For most enclosed helical or spur gearboxes in Australian industrial facilities, ISO VG 220 EP gear oil (GL-4) is the most common specification. Worm gearboxes require compounded gear oil or PAG synthetic — standard EP gear oil corrodes bronze worm wheels. Heavier-loaded or slower gearboxes may require ISO VG 320 or 460. If the specification is unavailable, contact the gearbox manufacturer.
What gear oil does a worm gearbox need?
Worm gearboxes with phosphor-bronze worm wheels cannot use standard sulphur-phosphorus EP gear oil (GL-4 or GL-5). The sulphur chemistry corrodes the bronze. Use compounded gear oil (AGMA Comp specification) — a mineral gear oil containing 3–10% fatty acid that provides lubrication for the bronze contact without chemical corrosion. ISO VG 220 Comp suits most standard worm gearboxes; ISO VG 320 Comp for slower, heavier drives. PAG synthetic gear oil is a higher-performance alternative with better film strength and reduced operating temperature, but it is not compatible with all seals and must not be mixed with mineral oil.
How often should I change industrial gearbox oil?
For most enclosed industrial gearboxes on mineral EP gear oil, change every 2,000–4,000 hours of operation or annually — whichever comes first. On synthetic PAO, extend to 4,000–8,000 hours or every two years. Always perform a first-change at 500–1,000 hours on a new gearbox to remove run-in metallic swarf. Change immediately after any water ingress event, seal failure, or sustained overtemperature. For critical gearboxes, oil analysis on a 1,000-hour cycle gives you the actual condition of the oil rather than relying on time-based estimates.
Can I use gear oil in a hydraulic system?
No. Gear oil contains EP additives — typically sulphur-phosphorus chemistry — that are corrosive to the copper and yellow metal alloys used in hydraulic pump internals (vane pump rotors, bronze port plates in piston pumps). EP chemistry would also degrade hydraulic seals formulated for AW hydraulic oil. Additionally, gear oil is typically far more viscous than the ISO VG 32, 46, or 68 grades used in hydraulic systems, causing pump cavitation and sluggish valve response. Use hydraulic oil in hydraulic systems and gear oil in gearboxes — they serve different applications with different additive requirements.
Can I mix different brands of gear oil?
Mixing gear oils of the same ISO VG grade and GL classification from different manufacturers is generally lower risk than mixing across categories, but it is still not recommended practice. Additive packages from different manufacturers may be incompatible — causing foaming, sludge formation, or additive precipitation that reduces the oil's performance. The safest approach is to drain, flush if practical, and refill with the new product. If topping up is unavoidable, use the same product or confirm compatibility with both manufacturers before mixing. Never mix mineral and synthetic gear oils, PAG with mineral or PAO, or compounded and standard EP gear oils.
Shop Gear Oils at AIMS Industrial
AIMS Industrial stocks industrial gear oil across the full viscosity range — ISO VG 100 through ISO VG 680, including compounded grades for worm gearboxes and synthetic options for demanding applications. Browse the full lubrication range at AIMS Industrial, including gear oil, hydraulic oil, compressor oil, chain lubricants, and way oil, available in 5L, 20L, and drum quantities for Australian industrial and maintenance operations.
For the full overview of industrial lubricant types and when to use each, see the AIMS Industrial Lubricants Guide. For hydraulic system lubrication, see the AIMS Hydraulic Oil Guide.
