What are the differences between HLP, HVLP, and HETG hydraulic fluids?

HLP, HVLP, and HETG are three distinct hydraulic fluid families. HLP is a mineral oil with anti-wear additives and a moderate viscosity index, making it the general-purpose workhorse for stable-temperature industrial systems. HVLP is the same anti-wear mineral oil base with added viscosity index improvers, so it holds its viscosity across wide temperature swings, which suits mobile and outdoor equipment. HETG is a biodegradable, vegetable-oil-based (triglyceride) fluid designed for environmentally sensitive applications, with excellent lubricity but tighter limits on temperature, water, and seal compatibility.

Why hydraulic fluid classification matters? 

A hydraulic fluid does far more than transmit pressure. It lubricates pumps and valves, carries heat away from working components, protects against corrosion, and helps seal clearances. Choosing the wrong fluid shows up as a sluggish response, overheating, accelerated wear, seal damage, or premature oil breakdown.

The codes HLP, HVLP, and HETG come from the standards that classify hydraulic fluids. HLP and HVLP are defined in DIN 51524 (Parts 2 and 3), and their ISO 6743-4 equivalents are HM and HV. HETG is defined in ISO 15380 for environmentally acceptable fluids. Understanding what each letter means makes it much easier to match a fluid to your machine, climate, and compliance needs.

What is HLP hydraulic fluid?

HLP is a refined mineral oil blended with additives for oxidation resistance, corrosion protection, and anti-wear performance. The "H" stands for hydraulic oil, "L" for added protection against corrosion and aging, and "P" for improved wear protection. In ISO terms it is known as HM.

Key properties of HLP

• Anti-wear protection: Typically uses zinc-based additives that form a protective film on metal surfaces under boundary lubrication.
• Moderate viscosity index: Usually around 95 to 105, which is adequate when operating temperatures stay fairly constant.
• Good oxidation and corrosion resistance: Supports long drain intervals in clean, well-maintained systems.
• Wide availability and low cost: The most economical of the three options.

Where HLP is used

HLP suits stationary industrial machinery such as presses, injection molding machines, machine tools, and power packs. These systems run in temperature-controlled environments, so the fluid does not need a high viscosity index. Common grades are ISO VG 32, 46, and 68.

What is HVLP hydraulic fluid?

HVLP starts with the same anti-wear mineral oil formulation as HLP, then adds viscosity index (VI) improver polymers. The extra "V" stands for viscosity-temperature behavior. In ISO terms it is called HV.

Key properties of HVLP

• High viscosity index: Typically 140 or above, meaning viscosity changes far less between cold start and hot operation.
• Better cold-start flow: Oil reaches pumps and actuators quickly in low temperatures, reducing cavitation and start-up wear.
• Stable viscosity when hot: Maintains film strength at high operating temperatures, protecting efficiency and components.
• Shear sensitivity: The polymer additives can be mechanically sheared in high-pressure pumps and valves, causing permanent viscosity loss over time.

Where HVLP is used

HVLP is the standard choice for excavators, loaders, cranes, forestry and agricultural machinery, and any outdoor equipment exposed to seasonal or daily temperature changes. It is also useful in indoor systems with large load-driven temperature swings. Because it costs more than HLP and can shear, it is worth specifying only where a wide temperature range truly exists.

What is HETG hydraulic fluid?

HETG is a biodegradable hydraulic fluid based on triglycerides, which are natural esters from vegetable oils such as rapeseed (canola). Under ISO 15380, HETG is one category of environmentally acceptable lubricants, alongside synthetic esters (HEES) and polyglycols (HEPG).

Key properties of HETG

• Rapid biodegradability and low toxicity: Reduces environmental damage if a hose bursts or a seal leaks.
• Excellent lubricity: Natural ester molecules cling strongly to metal, giving outstanding wear protection.
• Very high viscosity index: Often 190 or higher, so viscosity stays stable across temperature changes.
• High flash point: Safer than mineral oil in terms of fire risk, though it is not a fire-resistant fluid.

Limitations of HETG

• Lower oxidation stability: Triglycerides oxidize faster than mineral oil, so continuous operating temperatures are generally kept below about 70 to 80°C.
• Hydrolysis risk: Water contamination breaks the ester down and forms acids, so moisture control is essential.
• Limited low-temperature performance: Pour points are typically poorer than premium mineral oils.
• Higher cost: Often two to three times the price of mineral oil.
• Seal compatibility: NBR and FKM generally work well, but some polyurethanes can suffer hydrolysis. Always check with the seal supplier.

Where HETG is used

HETG is common in forestry machinery, marine and dredging equipment, hydroelectric plants, dam gates, agricultural equipment, and construction near rivers, lakes, groundwater, and other sensitive ecosystems. It is also used where regulations or customer contracts require environmentally acceptable lubricants.

HLP vs HVLP vs HETG: side-by-side comparison

How to choose the right fluid

Start with the operating environment. If the machine runs indoors at stable temperatures, HLP is almost always the most cost-effective answer. If it works outdoors, across seasons, or sees large temperature swings, HVLP protects efficiency and reduces cold-start wear. If a leak could reach soil or water, or regulations demand a biodegradable fluid, HETG is the right category.

Next, check the equipment manufacturer's specification, since pump and valve builders often publish approved fluid types and cleanliness targets. Then consider the following:

• Operating temperature: Keep HETG systems well within their thermal limits and add cooling if needed.
• Seal and hose materials: Confirm compatibility before switching fluid families.
• Contamination control: Water and particles shorten the life of every fluid, but HETG is especially sensitive to moisture.
• Total cost of ownership: A pricier fluid can pay back through longer component life, fewer breakdowns, and avoided cleanup costs.

Switching between fluids

Mixing fluid families is risky. Additive packages can clash, causing foaming, filter plugging, or additive dropout. Changing from HLP or HVLP to HETG requires a thorough flush, and standards typically recommend keeping residual mineral oil to a few percent at most so the fluid still meets its biodegradability claims. Replace filters, inspect seals, and run oil analysis after changeover to confirm the system is clean and stable.

HLP, HVLP, and HETG each solve a different problem. HLP offers dependable, affordable protection for stable industrial systems. HVLP adds temperature resilience for mobile and outdoor machinery. HETG brings environmental protection and strong lubricity where leaks carry ecological risk. Match the fluid to your temperature range, environment, and equipment approvals; maintain strict cleanliness, and regular oil analysis will keep whichever fluid you choose performing reliably.

1. What does HLP stand for in hydraulic oil?

HLP refers to a mineral-based hydraulic oil that contains additives for protection against corrosion and aging (L) and for improved wear protection (P). It is defined in DIN 51524 Part 2 and is equivalent to the ISO classification HM.

2. Is HVLP better than HLP?

Not always. HVLP is better when temperatures vary widely because of its higher viscosity index. For stable, temperature-controlled indoor systems, HLP performs equally well at a lower price and without the shear risk of viscosity improvers.

3. Is HETG hydraulic fluid fire resistant?

No. HETG has a higher flash point than many mineral oils, but it is not classified as a fire-resistant fluid. Systems near ignition sources still need approved fire-resistant fluids such as HFC or HFDU types.

4. Can I mix HLP, HVLP, and HETG fluids?

HLP and HVLP can often be topped up with each other in an emergency, though performance may change. Mixing mineral oils with HETG is not recommended because it can cause additive conflicts, foaming, and loss of biodegradability. Always flush when changing fluid families.

5. Which fluid is best for a hydraulic excavator?

Most excavators use HVLP because of changing weather and load-driven temperature swings. If the machine works near water or in environmentally protected areas, HETG or another environmentally acceptable fluid may be required, provided the manufacturer approves it.