What are the different grades of hydraulic oil?

Hydraulic oil is graded in two main ways. The first is viscosity grade, most commonly ISO VG 22, 32, 46, 68, 100, and 150, which tells you how thick the oil is at 40°C. The second is quality or type classification, such as HL, HM (HLP), HV (HVLP), and HG under ISO 6743-4 and DIN 51524, which tells you what additives the oil contains and what duty it can handle. Choosing the right grade means matching both: the correct viscosity for your temperature and pump and the correct quality level for your pressure and operating conditions.

Why do hydraulic oil grades matter?

Hydraulic oil does far more than transmit power. It lubricates pumps, valves, and cylinders; carries heat away from working components; seals internal clearances; and flushes out contaminants. Use a grade that is too thin, and you get internal leakage, wear, and reduced efficiency. Use one that is too thick, and you get a sluggish response, cavitation, heat, and wasted energy.

Because of this, "hydraulic oil" is not one product. It is a family of fluids, and grades exist to help engineers pick the right one for each system.

Grading by viscosity: ISO VG

Viscosity is the most important physical property of hydraulic oil. The ISO 3448 viscosity grade system (ISO VG) classifies industrial oils by their kinematic viscosity at 40°C, measured in centistokes (cSt). The grade number is the midpoint viscosity, with a tolerance of ±10%.

Common ISO VG grades

How to choose the right ISO VG

The pump manufacturer's recommendation is always the starting point. As a general rule, most pumps perform best when the oil viscosity at operating temperature sits between roughly 16 and 36 cSt, and many manufacturers specify an optimum near 25 to 36 cSt. Lower than that range increases leakage and wear. Higher than that range increases power loss and the risk of cavitation.

In practice:

• Cold climates or outdoor winter operation: ISO VG 22 or 32, ideally with a high viscosity index.
• Moderate conditions: ISO VG 32 or 46.
• Hot climates or high-temperature duty: ISO VG 46 or 68.

Viscosity index: How stable is the grade?

Viscosity changes with temperature. The viscosity index (VI) measures how much. A high VI means the oil thins out less as it heats up and thickens less as it cools down.

• Standard mineral oils (HL, HM): VI typically around 95 to 105.
• High-VI oils (HV/HVLP): VI of 140 or more, thanks to viscosity index improvers.

Equipment that runs outdoors, through wide temperature swings, or from cold start to full load benefits most from a high-VI grade. Excavators, cranes, and forestry machines are typical examples.

SAE viscosity grades

Some mobile equipment manufacturers specify hydraulic oil using SAE grades, such as SAE 10W, SAE 20W-20, or multigrade oils. SAE J300 grades are based on a different measurement method from ISO VG and are not directly interchangeable with it. When a manual gives an SAE grade, use a cross-reference chart or ask the oil supplier rather than assuming a match.

Grading by quality: HL, HM, HV, and HG

Viscosity tells you how thick the oil is. It says nothing about additive quality. That is where the ISO 6743-4 categories (and the related German DIN 51524 standard) come in.

HH: Plain mineral oil

HH oils have no additives. They are rarely used in modern hydraulic systems.

HL: Rust and oxidation inhibited

HL oils contain rust and oxidation inhibitors. They suit low-pressure systems where anti-wear protection is not critical, such as simple circulating systems and lightly loaded equipment.

HM (HLP): Anti-wear hydraulic oil

HM oils, called HLP in DIN 51524 Part 2, add anti-wear additives (often zinc-based) on top of HL properties. This is the most common quality level in industrial hydraulics. If your system has vane or piston pumps running at medium to high pressure, HM oil is usually the baseline requirement. Some systems, especially those with yellow-metal components, may need zinc-free anti-wear formulations, so always check pump and component specifications.

HV (HVLP): High viscosity index anti-wear oil

HV oils, called HVLP in DIN 51524 Part 3, have HM-level protection plus a high viscosity index. They are the standard choice for mobile hydraulics and outdoor machinery exposed to broad temperature ranges.

HG: anti-wear with stick-slip protection

HG oils add tackifiers to HM oils to reduce stick-slip in slow-moving machine tool slideways, where hydraulic and lubrication systems may share the same oil.

Fire-resistant hydraulic fluids

Where a leak could meet an ignition source, as in steel mills, foundries, die casting, and underground mining, standard mineral oil is a hazard. Fire-resistant fluids are graded under a separate ISO 6743-4 family:

• HFAE / HFAS: Oil-in-water emulsions or synthetic water solutions with very high water content. They offer excellent fire resistance but limited lubricity and temperature range.
• HFB: Water-in-oil emulsions, less common today.
• HFC: Water-glycol fluids, typically around 35 to 45% water, widely used where fire resistance is needed with reasonable lubricity.
• HFDR: Phosphate ester fluids with excellent fire resistance and lubricity, but with seal compatibility restrictions.
• HFDU: Other synthetic fluids, often polyol esters, offering fire resistance with better environmental profiles.

These fluids need compatible seals, paints, and filters and cannot generally be swapped in for mineral oil without a full system review.

Environmentally acceptable hydraulic fluids

For machinery operating near water, forests, farmland, or other sensitive environments, biodegradable options are specified under ISO 15380:

• HETG: Triglyceride (vegetable oil-based)
• HEES: Synthetic esters
• HEPG: Polyglycols
• HEPR: Polyalphaolefins and related hydrocarbons

They offer good lubricity and biodegradability but require attention to oxidation stability, operating temperature limits, and seal compatibility. Changing over from mineral oil generally requires thorough flushing.

Manufacturer approvals

Beyond ISO and DIN, many pump and equipment manufacturers publish their own approval specifications. These commonly cover anti-wear performance, filterability, water tolerance, and thermal stability. Even if two oils are both labelled ISO VG 46 HM, they may not meet the same OEM requirements. Where an approval is required, the oil datasheet should list it explicitly.

Practical tips for choosing and managing oil grades

1. Start with the OEM manual. Pump and equipment makers specify both viscosity and quality level.
2. Check operating temperature, not just ambient temperature. Reservoir temperature often runs well above ambient.
3. Match quality to pressure. Higher-pressure systems with piston or vane pumps generally need HM or HV oils.
4. Do not mix oil types casually. Different additive packages or base oil families can cause additive dropout, foaming, or seal problems.
5. Keep it clean. Even the right grade fails early with poor contamination control. Follow ISO 4406 cleanliness targets for your components.
6. Monitor with oil analysis. Regular sampling shows viscosity drift, additive depletion, and wear metals before failure occurs.

1. What is the most common hydraulic oil grade?

ISO VG 46 with HM (anti-wear) quality is the most widely used grade in general industrial and mobile hydraulic systems. ISO VG 32 and ISO VG 68 are the next most common, depending on climate and duty.

2. Can I use a different viscosity grade than the manufacturer recommends?

Only within the limits the manufacturer allows. A slightly different grade may be acceptable if it keeps operating viscosity within the pump's specified range, but a wrong choice can cause wear, overheating, or cavitation. Confirm with the OEM before changing.

3. What is the difference between HL, HM, and HV hydraulic oil?

HL oils have rust and oxidation inhibitors only. HM oils add anti-wear additives for higher-pressure duty. HV oils are HM oils with a high viscosity index, giving more stable viscosity over wide temperature ranges.

4. Is ISO VG the same as SAE grade?

No. ISO VG measures viscosity at 40°C, while SAE J300 grades use different test methods and conditions. They cannot be converted directly, so use manufacturer cross-reference charts.

5. Can different grades of hydraulic oil be mixed?

Mixing different viscosity grades of the same oil type is sometimes tolerable in emergencies, but it changes the final viscosity. Mixing different quality types or base oil families, such as mineral and synthetic, or mineral and fire-resistant fluids, can cause serious compatibility problems and should be avoided unless the supplier confirms it is safe.