Hydraulic Oil Guide: ISO VG Grades, AW vs HV & Compatibility

Hydraulic Oil vs Hydraulic Fluid — What's the Difference?

The terms are often used interchangeably in Australian workshops and on product shelves, but they are not technically the same thing. Hydraulic fluid is the broad category — any fluid used to transmit power in a hydraulic system. Hydraulic oil is a specific type of hydraulic fluid: a mineral oil-based product refined from petroleum and formulated with additive packages for anti-wear protection, oxidation resistance, rust and corrosion inhibition, and foam suppression.

Hydraulic system leaks at the fittings often trace back to a missing or worn flat-face seal rather than a hose or pump failure — for BSP/JIC fitting sealing the Dowty washer (bonded seal) is the standard solution. See the Dowty washer and bonded seal guide for BSP-G size selection and elastomer choice (NBR, Viton or EPDM).

In practice, the vast majority of hydraulic systems in Australian industry, construction, agriculture, and workshop equipment run on mineral hydraulic oil. Water-glycol fluids and synthetic hydraulic fluids exist for specialised applications — high fire-risk environments, extreme temperature ranges — but if you are buying fluid for a workshop press, a pallet jack, a log splitter, an agricultural tractor, or a piece of construction plant, you are almost certainly looking for mineral hydraulic oil.

This guide focuses on mineral hydraulic oil: how the grading system works, how to select the right grade, and how to avoid the specification errors that damage pumps, seals, and systems.

Understanding ISO VG: What the Number Means

The ISO Viscosity Grade (ISO VG) system is the global standard for classifying lubricating oil viscosity. Every hydraulic oil sold in Australia carries an ISO VG rating — typically 32, 46, 68, 100, or 150. Understanding what the number means is essential to making a correct selection.

ISO VG = Kinematic Viscosity at 40°C

The ISO VG number represents the oil's nominal kinematic viscosity in centistokes (cSt) measured at 40°C. The standard allows a ±10% tolerance around the nominal value:

For a cross-reference of the complete ISO VG grade range — all 20 grades from VG 2 to VG 3200 — alongside SAE engine oil and SAE gear oil equivalents, see the ISO VG viscosity chart.

Higher number = thicker oil at 40°C. A thicker oil provides greater film strength at elevated temperatures and loads, but flows more slowly at cold start and creates greater pressure drop through system components. A thinner oil flows freely at low temperatures and speeds but provides less film protection at high loads and temperatures.

What ISO VG Does Not Tell You

The ISO VG number only describes viscosity at one temperature — 40°C. It tells you nothing about how the oil behaves at cold start (say, on a winter morning in inland NSW) or at elevated operating temperatures. That is where the Viscosity Index (VI) becomes important — covered in the HVI section below.

The Four Types of Hydraulic Fluid

While mineral hydraulic oil covers most applications, there are four broad categories of hydraulic fluid, each suited to different operating conditions and risk profiles.

1. Mineral Oil-Based Hydraulic Oil

The standard for the vast majority of industrial and mobile hydraulic equipment. Refined from petroleum base stocks and formulated with additive packages covering anti-wear (AW), oxidation inhibition, rust and corrosion protection, and foam suppression. Available in ISO VG grades 22 through 150. Cost-effective, widely available in Australia through industrial suppliers and lubricant specialists, and compatible with standard hydraulic seals and materials. The default choice unless the equipment or application specifically requires otherwise.

2. Synthetic Hydraulic Fluid

Polyalphaolefin (PAO) or ester-based fluids with superior temperature range performance, higher natural Viscosity Index, better oxidation stability, and longer service life than mineral oil. Used where mineral oil is inadequate — extreme cold start conditions, very high operating temperatures, extended drain intervals, or high-performance equipment with tight tolerances. Significantly more expensive than mineral oil. Compatible with most standard hydraulic seals but check for compatibility with polyurethane seals, which can swell with some ester-based synthetics.

3. Water-Glycol Hydraulic Fluid

Fire-resistant fluid consisting of water and glycol with additives, typically 35–50% water content. Used in applications with ignition risk — die-casting, steel mills, coal mining, and anywhere a hydraulic line failure could expose fluid to an ignition source. Water-glycol is significantly less lubricating than mineral oil and requires careful system management: concentration monitoring, pH control, and compatibility checks with system metals (zinc, cadmium, and magnesium are generally incompatible). Never mix with mineral oil.

4. Phosphate Ester and Other Fire-Resistant Synthetic Fluids

Highly fire-resistant synthetic fluids used in aviation, power generation, and some industrial applications. Phosphate esters are aggressive solvents — they are incompatible with standard nitrile seals, most paints, and many plastics. Systems using these fluids require specific seal materials (ethylene propylene or Viton), stainless steel or special alloy components, and careful handling procedures. Rarely encountered in standard Australian workshop or industrial maintenance — specified applications only.

AW Hydraulic Oil: Anti-Wear Explained

Most hydraulic oil sold for industrial and workshop use in Australia is labelled AW — Anti-Wear. This is not merely a marketing claim; it refers to a specific additive package, most commonly based on zinc dialkyldithiophosphate (ZDDP), that forms a sacrificial protective film on metal surfaces under high-pressure contact.

The components most dependent on AW protection are hydraulic pumps — particularly vane pumps, piston pumps, and gear pumps — where metal-to-metal contact occurs at high pressure. Without AW additives, these contact zones experience accelerated wear, shortening pump life significantly.

AW hydraulic oil is available across all ISO VG grades and is the appropriate choice for most applications. If your system specification says "AW hydraulic oil" — or simply lists an ISO VG grade without further qualification in a general industrial context — AW mineral oil is correct.

Zinc-Based vs Zinc-Free AW Hydraulic Oil

ZDDP-based AW additives contain zinc. Most AW hydraulic oil is zinc-based. However, some hydraulic systems — particularly those using silver-plated components, certain pump types from specific manufacturers, or systems with silver-containing filters — require zinc-free hydraulic oil. Zinc can attack silver under certain conditions, leading to corrosion of silver-plated components.

Zinc-free hydraulic oils use ashless (non-metallic) anti-wear additive chemistry instead. Unless your equipment documentation specifically calls for zinc-free hydraulic oil, standard zinc-based AW oil is correct. Check the OEM specification if in doubt.

HVI Hydraulic Oil: High Viscosity Index Explained

Viscosity Index (VI) measures how much an oil's viscosity changes as temperature changes. A high VI means the oil maintains relatively consistent thickness across a wide temperature range — it does not become dangerously thin at high temperature or excessively thick at low temperature.

• Standard mineral hydraulic oil: VI of approximately 95–100
• HVI hydraulic oil: VI of 140–160 or higher

HVI hydraulic oil is produced either through deep hydrocracking of base stocks (Group III) or through addition of VI improver polymer additives. It is the correct choice for equipment that:

• Operates across a wide ambient temperature range — outdoor plant in Australian conditions that experience cold mornings and hot afternoons
• Cycles through cold start and high operating temperature in the same shift
• Is operated across different Australian climate zones (tropical north versus temperate south)
• Requires a single oil specification across a fleet with diverse operating environments

Brand examples available in Australia: Gulf Western Superdraulic HVI, Castrol Hyspin AWS, Mobil DTE 10 Excel series. For equipment that operates in a stable temperature environment — an indoor factory, for example — standard mineral AW hydraulic oil is adequate and more cost-effective than HVI.

ISO VG 32 vs 46 vs 68: Which Grade for Your Application?

Selecting the wrong ISO VG grade is one of the most common specification errors in Australian hydraulic maintenance. Too thin and you get internal leakage, heat generation, and reduced system efficiency. Too thick and you get sluggish response, cavitation on cold start, increased filter pressure drop, and potential pump damage. Always check the OEM specification first — this table is a guide only.

46 or 68: Answering Australia's Most Searched Hydraulic Oil Question

This is consistently the most-asked hydraulic oil question in Australian workshops, and the answer has a clear logic to it.

Use ISO VG 46 if:

• Your equipment manual specifies VG 46 (this is the most common specification for Australian industrial and mobile equipment)
• You operate in moderate ambient temperatures — most of temperate and southern Australia year-round
• Your equipment is relatively modern with close pump tolerances
• You are unsure and need a safe general-purpose choice for most workshop or agricultural hydraulic applications

Use ISO VG 68 if:

• Your equipment manual specifies VG 68
• You operate in consistently hot ambient conditions — far north Queensland, outback NT, or extended summer operation in hot sheds
• Your system runs at high pressure (above 250 bar) where a thicker film provides better protection
• Your equipment is older with worn pump or cylinder clearances that benefit from additional film thickness
• Your system runs hot even in moderate climates — the reservoir runs warm to the touch after normal operation

If you genuinely cannot determine which is correct, ISO VG 46 is the safer default for Australian conditions — it suits more applications and is less likely to cause cold-start problems in temperate and cool climates.

How to Read a Hydraulic Oil Data Sheet

Every commercial hydraulic oil has a Product Data Sheet (PDS) available from the manufacturer. Knowing which figures matter helps you make direct comparisons between products and confirm suitability for your application.

Australian Hydraulic Oil Brands: What's Available

Several brands dominate the Australian hydraulic oil market, with products widely available through industrial suppliers, automotive trade outlets, and agricultural merchants.

Hydraulic Oil Change Intervals and Contamination

Change Intervals

Hydraulic oil does not wear out from use in the same way as engine oil — the main degradation mechanisms are oxidation (from heat and oxygen exposure), additive depletion, and contamination ingress. Published intervals are guides only; actual condition determines timing:

• Light-duty indoor industrial equipment: 2,000–4,000 hours or annually, whichever comes first
• Mobile plant and construction equipment: 1,000–2,000 hours or per OEM schedule
• High-contamination environments (mining, earthmoving, tropical conditions): More frequently; use oil analysis to determine actual interval
• Small workshop equipment (presses, splitters, jacks): Annually or when contamination is evident

Signs the Oil Needs Changing

• Milky or cloudy appearance: Water contamination — from condensation, seal failure, or coolant ingress. Change immediately.
• Dark brown or black colour with burnt odour: Oxidation and thermal degradation — oil has overheated or is long past service life.
• Metallic sheen or visible particles: Solid contamination — wear debris, filter failure, or external ingress through breathers or seals.
• Persistent foaming during operation: Air ingress, contamination, or additive depletion. Foaming reduces lubrication significantly.
• Sluggish response or reduced system performance: Viscosity breakdown or gross contamination causing internal leakage.

Filtration and Contamination Control

Hydraulic system contamination — primarily particulate — is the leading cause of pump and valve wear in Australian industrial equipment. Key contamination control practices:

• Replace hydraulic filters at the OEM-specified interval, not just when the bypass indicator trips
• Use a breather filter or desiccant breather on the reservoir — open breathers allow dust and moisture ingress
• Cap all open hydraulic fittings immediately when disconnecting lines
• Pre-filter new oil before adding to the system — new oil from drums is not clean enough for precision hydraulic circuits
• Investigate any sudden increase in system temperature — it often indicates a developing contamination or efficiency problem before it becomes a failure

5 Common Hydraulic Oil Mistakes

1. Using the Wrong ISO VG Grade

The most common specification error. Using VG 68 where VG 46 is specified causes sluggish operation, cavitation on cold start, and increased filter loading. Using VG 46 where VG 68 is specified causes internal leakage, heat generation, and accelerated wear at high temperatures. Always start with the service manual.

2. Mixing Incompatible Fluids

Mixing mineral oil with water-glycol or phosphate ester fluid causes immediate chemical incompatibility. Mixing different brands' mineral oils is usually — but not always — safe; mixing zinc-based and zinc-free additive packages can cause sludge formation. When in doubt, drain and flush before refilling with the new fluid.

3. Ignoring Water Contamination

A milky or hazy appearance means water is present. Water in hydraulic oil causes corrosion, promotes microbial growth, reduces lubrication film strength, and can cause cavitation damage. This does not resolve itself — the system must be drained, the source of water ingress identified and sealed, and the reservoir cleaned before refilling.

4. Running Past Change Interval

Hydraulic oil that has oxidised loses its additive package — anti-wear additives deplete, foam inhibitors break down, and the base oil thickens. Running oxidised oil causes accelerated pump wear that is not visible until the pump fails. Oil analysis is inexpensive; pump replacement is not.

5. Adding Oil Without Identifying Why It Was Low

Hydraulic systems are closed circuits — they do not consume oil. If the reservoir level is dropping, oil is going somewhere: a leaking seal, a weeping fitting, a ruptured hose, or a cylinder rod wiper failure. Adding oil without finding the leak masks a developing failure. Investigate the source before topping up.

Hydraulic Jack Oil: A Note for Workshop Use

Hydraulic bottle jacks, trolley jacks, and floor jacks are common in Australian workshops, and hydraulic jack oil is frequently purchased as a separate product. In most cases, hydraulic jack oil is simply a light mineral hydraulic oil — typically equivalent to ISO VG 15 or ISO VG 22 — formulated for the low-pressure, small-volume circuits found in portable lifting equipment.

Standard ISO VG 32 or 46 industrial hydraulic oil should not be used in most bottle jacks and trolley jacks without checking the manufacturer's specification. The heavier viscosity can cause sluggish ram extension and may prevent the jack from reaching full lift height, particularly in cooler conditions. Dedicated hydraulic jack oil is available in small quantities from automotive and industrial suppliers and is the correct product for these applications.

Conversely, do not use hydraulic jack oil in industrial hydraulic systems — it lacks the AW additive package required for pump protection and is too thin for most system designs.

Storage and Handling of Hydraulic Oil

Hydraulic oil contamination can occur before it even enters a system if storage and handling practices are poor. This is an underappreciated source of system problems in Australian workshops and plant operations.

• Store drums horizontally or bung-side down when possible — vertical storage with bungs facing up allows water ingress through temperature cycling as the drum breathes
• Keep drums in a covered, dry area — UV exposure degrades additive packages over time; moisture ingress from rain causes the contamination problems described above
• Label drums clearly with product name, ISO VG grade, and date received — mixing grades due to poor labelling is a preventable error
• Use dedicated transfer equipment — pumps, hoses, and funnels used for hydraulic oil should not be shared with other lubricants; residual gear oil or engine oil left in transfer equipment contaminates the hydraulic fluid
• Pre-filter oil from drums before adding to systems — new oil from bulk drums can contain particulate contamination from manufacturing and transport; passing it through a 10-micron filter before use removes this before it reaches system components
• Check shelf life — mineral hydraulic oil has a typical shelf life of 3–5 years in unopened, correctly stored containers; additive packages degrade over time even without contamination

Frequently Asked Questions

What is the difference between hydraulic oil and hydraulic fluid?

Hydraulic fluid is the broad term for any fluid used to transmit power in a hydraulic system — it includes mineral oils, synthetic fluids, water-glycol mixtures, and fire-resistant fluids. Hydraulic oil is a specific category within that group: mineral oil-based fluid refined from petroleum and formulated with additive packages for anti-wear, oxidation resistance, and corrosion protection. In Australian workshop and industrial use, 'hydraulic oil' and 'hydraulic fluid' are often used interchangeably, but technically hydraulic oil refers to the mineral oil type.

What are the 4 types of hydraulic fluid?

The four main categories are: (1) Mineral oil-based hydraulic oil — the most common type, available in ISO VG grades 32 to 150, used in the vast majority of industrial and mobile equipment; (2) Synthetic hydraulic fluid — polyalphaolefin (PAO) or ester-based, used where wide temperature range or extended service life is required; (3) Water-glycol fluid — fire-resistant, used in applications near ignition sources such as die-casting and steel mills; and (4) Phosphate ester fluid — highly fire-resistant, used in aviation and power generation. Most Australian workshop and industrial equipment uses mineral oil-based hydraulic oil.

What does ISO VG mean on hydraulic oil?

ISO VG stands for International Organisation for Standardisation Viscosity Grade. The number (32, 46, 68, 100, 150) represents the oil's kinematic viscosity in centistokes (cSt) measured at 40°C. A higher number means a thicker oil at that temperature. ISO VG 46 has a nominal viscosity of 46 cSt at 40°C. The grade tells you how the oil flows under normal operating conditions — not how it performs at cold start or at elevated temperature, which is where the Viscosity Index (VI) rating becomes important.

What is thicker — ISO VG 32 or ISO VG 46 hydraulic oil?

ISO VG 46 is thicker (higher viscosity) than ISO VG 32. The ISO VG number represents kinematic viscosity in centistokes at 40°C — so 46 cSt is thicker than 32 cSt. In practical terms, ISO VG 32 flows more freely and is preferred for systems operating in cold conditions or at high speeds with low pressure. ISO VG 46 provides greater film thickness and is better suited to higher load and higher temperature applications. Always check your equipment's OEM specification before selecting a grade.

Should I use ISO VG 46 or ISO VG 68 hydraulic oil?

The correct choice depends on your equipment's OEM specification, operating temperature, and system pressure. As a general guide: ISO VG 46 suits most general industrial hydraulic systems, mobile equipment operating in moderate Australian conditions (15–40°C ambient), and most agricultural machinery. ISO VG 68 suits equipment operating in higher ambient temperatures, higher pressure systems, or older equipment with wider clearances that benefits from a thicker film. Always check the machine's service manual first — using the wrong grade can reduce efficiency, increase heat generation, and shorten pump and seal life.

Can I mix hydraulic oils of different grades?

Mixing grades is not recommended. Blending ISO VG 46 and ISO VG 68, for example, will produce an intermediate viscosity that may not match your system's design requirements. More critically, mixing oils from different manufacturers or different additive packages (such as zinc-based and zinc-free) can cause additive incompatibility, resulting in sludge formation, foaming, or accelerated wear. If a top-up is unavoidable, use the same brand and grade and flush the system at the earliest opportunity. Never mix mineral oil with water-glycol or synthetic hydraulic fluids.

Does it matter what hydraulic oil I use?

Yes — using the wrong hydraulic oil is one of the most common causes of hydraulic system failure in Australian workshops. The viscosity grade affects pump efficiency, heat generation, and cavitation risk. Additive compatibility affects seal integrity and corrosion protection. Using too thin an oil causes excessive internal leakage and heat; too thick an oil causes sluggish response, cavitation on cold start, and increased pressure drop across filters. Always use the grade and type specified by the equipment manufacturer.

What hydraulic oil should I use for a log splitter, tractor, or excavator?

For most Australian log splitters and small hydraulic equipment: ISO VG 46 AW (anti-wear) mineral hydraulic oil is the standard recommendation. For tractors with combined hydraulic and transmission systems (common on farm equipment): check whether the machine requires a Universal Tractor Transmission Oil (UTTO) rather than a straight hydraulic oil — many tractor hydraulic circuits share fluid with the gearbox and require UTTO. For excavators and construction plant: ISO VG 46 or 68 depending on ambient temperature and OEM specification — always follow the service manual.

What is AW hydraulic oil?

AW stands for Anti-Wear. AW hydraulic oil contains zinc dialkyldithiophosphate (ZDDP) or similar additives that form a protective film on metal surfaces under high-pressure contact — particularly vane pump rotors, piston pump shoes, and gear pump teeth. AW hydraulic oil is the standard for most industrial and mobile hydraulic systems. It is available across all ISO VG grades. Most hydraulic oil sold in Australia for workshop and industrial use is AW-rated unless specifically labelled otherwise.

What is HVI hydraulic oil?

HVI stands for High Viscosity Index. Viscosity Index (VI) measures how much a fluid's viscosity changes with temperature — a higher VI means the oil maintains a more consistent viscosity across a wide temperature range. Standard mineral hydraulic oil has a VI of around 95–100. HVI oil has a VI of 140–160+, achieved through either highly refined base stocks or VI improver additives. HVI hydraulic oil is used in equipment that operates across wide temperature ranges — outdoor plant in Australian conditions that experience cold mornings and hot afternoons, or equipment moved between climate zones.

How often should hydraulic oil be changed?

Service intervals vary by equipment type, duty cycle, and operating conditions. General guidelines for mineral hydraulic oil: light-duty industrial equipment — every 2,000 hours or annually; mobile plant and construction equipment — every 1,000 hours or per OEM specification; high-contamination environments (mining, earthmoving) — more frequently based on oil analysis. The most reliable method is oil analysis — testing viscosity, contamination level, acid number, and wear metal content determines actual oil condition rather than guessing based on hours. Replace regardless of hours if the oil appears milky (water contamination), dark and thick (oxidation), or has a burnt smell.

How do I know if my hydraulic oil is contaminated?

Key indicators of contaminated hydraulic oil: milky or cloudy appearance (water ingress — common after condensation or seal failure); dark brown or black colour with a burnt smell (oxidation — oil has overheated or is well past service life); visible particles or metallic sheen (solid contamination — wear debris, seal fragments, or external ingress); foaming during operation (air contamination or additive depletion); sluggish system response or reduced lifting capacity (viscosity breakdown or gross contamination). When in doubt, send a sample for oil analysis — this is the definitive test and is inexpensive relative to the cost of a pump replacement.

Shop Hydraulic Oil at AIMS Industrial

AIMS Industrial stocks a range of hydraulic oils in ISO VG 32, 46, and 68 grades from leading Australian and international brands, available in 5L, 20L, and drum quantities. For dispensing hydraulic oil from 205L drums into machine reservoirs cleanly, see the Oil Pump & Drum Pump Guide — air-operated 3:1 or 5:1 ratio pumps are the workshop standard for hydraulic oil throughput. Browse the hydraulic oil range for current stock, or visit the full lubrication collection for greases, gear oils, and specialty lubricants. For equipment that relies on hydraulic systems, see our pallet jack guide covering hydraulic pallet jack selection and maintenance.