What Hydraulic Oil to Use in a Hydraulic Power Unit?

What Hydraulic Oil Is — and Why Choosing the Right One Matters

Hydraulic oil is a mineral or synthetic fluid that transmits power, lubricates internal components, and dissipates heat inside a hydraulic system. In a hydraulic power unit, it is not a passive medium — it is the working substance that determines how efficiently force is generated, how long seals and pumps last, and how reliably the entire system performs under load.

The short answer to "what hydraulic oil should I use" depends on three factors: the viscosity grade required by your pump, the operating temperature range of your application, and the additive package compatibility with your seals and metals. Get these right, and the hydraulic power unit runs cleanly for thousands of hours. Get them wrong, and you face cavitation, seal swell, and premature pump failure.

The Main Types of Hydraulic Oil and What They Contain

Most hydraulic oils fall into one of four categories. Each has a specific composition designed for different operating conditions.

Mineral-Based Hydraulic Oil (HL, HM, HV)

Refined from crude petroleum, these are the most common fluids used in standard hydraulic power units. ISO grades HL (rust and oxidation inhibitors), HM (anti-wear additives added), and HV (high viscosity index for wide temperature range) cover the majority of industrial and mobile applications. HM46 and HM68 are among the most widely specified grades globally.

Synthetic Hydraulic Oil (HFDU, PAO-based)

Polyalphaolefin (PAO) and ester-based synthetic fluids offer a wider operating temperature range, typically −40°C to +120°C, compared to mineral oil's practical range of roughly −20°C to +90°C. They are used in hydraulic power units operating in extreme cold (outdoor equipment in Nordic climates) or continuous high-cycle applications where thermal degradation is a concern.

Fire-Resistant Hydraulic Fluids (HFA, HFB, HFC, HFDU)

Used in foundries, die casting operations, and underground mining where leaks near ignition sources create fire risk. Water-glycol fluids (HFC) are common and typically run at 35–50% water content, which reduces lubricity — so pump pressure ratings must be de-rated accordingly.

Biodegradable Hydraulic Oil (HETG, HEPG, HEES)

Vegetable oil (HETG) and synthetic ester (HEES) fluids are required in environmentally sensitive areas — forestry machinery, marine deck equipment, agriculture near waterways. HEES fluids generally offer better oxidation stability than HETG and are more compatible with standard sealing materials.

Viscosity Grades Explained — What the Numbers Mean

ISO VG (Viscosity Grade) numbers represent the fluid's kinematic viscosity in centistokes (cSt) at 40°C. Common grades used in hydraulic power units are listed below:

VG 46 is the default choice for most hydraulic power units operating between 20°C and 50°C ambient temperature. If the system runs cooler consistently, step down to VG 32. If it runs hotter or at higher sustained pressure (above 250 bar), move up to VG 68.

Key Properties That Determine Oil Performance in a Hydraulic Power Unit

Viscosity grade is the starting point, but several other properties directly affect how the oil behaves inside the pump, valve block, and actuators of a hydraulic power unit.

• Viscosity Index (VI): Measures how much viscosity changes with temperature. A higher VI means more stable performance across temperature swings. Standard mineral HM oils sit around VI 95–105. HV oils and synthetics reach VI 150+.
• Anti-wear (AW) additives: Zinc dialkyldithiophosphate (ZDDP) is the most common. It forms a sacrificial film on pump vanes, gear teeth, and piston slippers. Essential for vane and gear pumps operating above 100 bar.
• Oxidation stability: Hydraulic oil degrades through oxidation, forming sludge and acids. Quality HM oils have an oxidation life of 2,000–4,000 hours in standard duty cycles. Synthetics extend this to 8,000+ hours.
• Demulsibility: The oil's ability to separate from water contamination. Poor demulsibility leads to emulsification, which destroys the oil film and accelerates corrosion inside the hydraulic power unit reservoir.
• Air release and foam resistance: Entrained air causes cavitation. Hydraulic oil should release dissolved air rapidly — a good test value is less than 5 minutes air release time at 50°C per ISO 9120.

How to Match Hydraulic Oil to Your Hydraulic Power Unit Pump Type

Different pump designs place different demands on the hydraulic fluid. Using the wrong oil type for the pump can cause wear rates far above design expectations.

Gear Pumps

Gear pumps are tolerant of a wide range of hydraulic oils. HM46 is the standard recommendation. They can handle slightly higher contamination levels than vane or piston pumps, but cleanliness to ISO 4406 class 18/16/13 or better is still advisable.

Vane Pumps

Vane pumps are the most sensitive to oil quality. They require good anti-wear protection because the vane tips run against the cam ring under spring and centrifugal force. Some vane pump manufacturers (Vickers/Eaton, for example) specify fluids meeting their own anti-wear test (the Vickers 35VQ25 test) rather than just ISO classification.

Axial Piston Pumps

High-pressure axial piston pumps (operating at 250–450 bar) in hydraulic power units used for presses, injection molding, and mobile cranes require excellent lubricity and cleanliness. Target ISO 4406 cleanliness code of 16/14/11 or better. HV46 or HV68 is commonly specified to handle the temperature variations in mobile applications.

Contamination: The Real Cause of Most Hydraulic Oil Failures

Studies from major hydraulic component manufacturers consistently show that over 70% of hydraulic system failures are caused by fluid contamination rather than oil degradation alone. Contamination comes in three forms:

• Particle contamination: Metal wear particles, casting sand, weld spatter, and dirt ingested through breathers or cylinder rod seals. Controlled by return-line and offline filtration, targeting 10-micron absolute filters or finer.
• Water contamination: Condensation in reservoirs with large thermal cycling, or external ingress. Water content above 0.1% begins to reduce the oil's load-carrying capacity and promotes bacterial growth in biodegradable fluids. Monitor with Karl Fischer titration.
• Chemical contamination: Mixing incompatible hydraulic fluids — for example, adding zinc-free (ashless) oil to a system containing zinc-based AW oil — can cause additive precipitation and filter plugging within hours.

When changing oil type or brand in an existing hydraulic power unit, always flush the system with a low-viscosity mineral oil and drain completely before refilling. Reservoir cleaning between oil changes is not optional — it removes sludge that contaminates fresh oil immediately.

Oil Change Intervals and Condition Monitoring

Changing hydraulic oil on a fixed-hour schedule regardless of actual condition wastes oil and risks either over-extending degraded fluid or discarding usable oil. Condition-based oil management is a better approach for hydraulic power units in continuous industrial use.

As a baseline, typical oil change intervals for mineral HM oil in a well-maintained hydraulic power unit are:

• Light duty, controlled environment: 2,000–4,000 hours
• Medium duty, moderate temperature cycling: 1,000–2,000 hours
• High duty, elevated temperature or contamination exposure: 500–1,000 hours

Condition indicators to test during scheduled oil sampling include: viscosity at 40°C (±10% deviation from new oil triggers review), acid number (TAN above 2.0 mg KOH/g for mineral oil indicates oxidation), water content (above 0.1% requires immediate action), and particle count per ISO 4406.

Practical Selection Guide: Which Hydraulic Oil to Use

If your hydraulic power unit manufacturer's manual specifies an oil type and viscosity — follow it exactly. That specification accounts for pump clearances, seal materials, and system pressure. When the manual is unavailable or gives only a broad range, use this decision path:

1. Identify your pump type (gear, vane, piston) and its maximum operating pressure.
2. Determine your normal operating temperature range at the reservoir.
3. If ambient temperature is stable and moderate (15°C–45°C): use HM46 for gear and vane pumps, HM68 for high-pressure piston pumps.
4. If ambient temperature swings widely or the unit is mobile: use HV46 for its higher viscosity index.
5. If the application is near water or ecologically sensitive areas: use HEES synthetic ester biodegradable fluid in the correct viscosity grade.
6. If fire risk is present: consult your pump manufacturer for approved fire-resistant fluid types and apply the relevant pressure de-rating.

Never mix hydraulic oil brands or types without confirming additive compatibility with both manufacturers. Even two HM46 oils from different suppliers can contain incompatible additive chemistry.