What is the difference between AW32, AW46, and AW68 hydraulic oil?

The difference between AW32, AW46, and AW68 hydraulic oil is viscosity. All three are anti-wear (AW) mineral-based hydraulic oils with similar additive packages, but the number is the ISO viscosity grade (VG), which is the oil's thickness in centistokes (cSt) at 40°C. AW32 is the thinnest, AW46 sits in the middle, and AW68 is the thickest. The right grade depends on your pump manufacturer's recommendation, operating temperature, and ambient climate. Choosing the wrong grade is one of the most common and most avoidable causes of hydraulic system problems. Oil that is too thin accelerates wear and leakage, while oil that is too thick causes sluggish response, heat, and cavitation. This guide explains what the codes mean, how the three grades compare, and how to choose between them.

What does "AW" mean in hydraulic oil?

"AW" stands for anti-wear. These oils are formulated with a base oil (usually a mineral oil) plus an additive package that typically includes:

• Anti-wear additives, commonly zinc-based (ZDDP) or ashless alternatives, that form a protective film on metal surfaces under boundary lubrication conditions
• Rust and corrosion inhibitors to protect steel components from moisture
• Oxidation inhibitors to slow oil breakdown at high temperatures
• Anti-foam and demulsifying agents to release air and separate water

AW oils are classified as HM fluids under ISO 6743-4 and are specified in DIN 51524 Part 2. They are the standard choice for vane, gear, and piston pumps in industrial and mobile equipment.

What do the numbers 32, 46, and 68 mean?

The number is the ISO VG (Viscosity Grade) defined in ISO 3448. It represents the midpoint kinematic viscosity of the oil at 40°C, measured in cSt (mm²/s). Each grade allows a tolerance of ±10%.

The 100°C and viscosity index (VI) figures vary between brands, so check the product data sheet. The key takeaway is that each step up in grade means a noticeably thicker oil at any given temperature.

AW32 hydraulic oil

AW32 is the lightest of the three grades. It flows easily, which makes it a strong fit for cooler environments and systems that start cold.

Typical applications

• Indoor industrial machinery in temperature-controlled plants
• Equipment in cold or temperate climates
• Systems with high-speed, low-load pumps

Advantages

• Easier cold starts and faster pump priming
• Lower internal fluid friction, so less energy loss in some systems

Limitations

• In hot conditions, viscosity can fall too low, thinning the lubricating film
• Higher risk of internal leakage across worn pumps and valves

AW46 hydraulic oil

AW46 is the most widely used hydraulic oil grade worldwide. It offers a balance between protection and flow, and it is the default recommendation in many OEM manuals.

Typical applications

• General industrial hydraulic systems and power packs
• Hydraulic presses and lifting equipment
• Mobile equipment such as excavators and loaders in moderate climates

Advantages

• Good all-round film strength across a wide operating temperature window
• Compatible with most pump types

Limitations

• Can be slightly too thick for very cold starts
• Can be slightly too thin for continuous high-temperature, high-load duty

AW68 hydraulic oil

AW68 is the heaviest of the three. It maintains a thicker lubricating film at high temperatures, which is why it is chosen for hot climates and demanding duty cycles.

Typical applications

• Equipment operating in hot climates or high ambient temperatures
• Heavy-duty mobile and industrial machinery working under high load
• Systems with high operating temperatures where cooling is limited

Advantages

• Stronger lubricating film at high temperatures
• Reduced internal leakage in worn components
• Better protection under heavy loads

Limitations

• Slow flow at cold start, which risks pump cavitation
• Greater energy consumption due to fluid friction
• More heat generation if the system is not designed for this grade

Side-by-side comparison

How to choose the right grade?

1. Follow the OEM recommendation first

Pump and equipment manufacturers specify an acceptable viscosity range at operating temperature. This is the most important input. Using a grade outside the recommended range can reduce component life and may affect warranty coverage.

2. Match viscosity to operating temperature

Most hydraulic pumps perform best when oil viscosity at operating temperature falls between roughly 16 and 36 cSt, with many manufacturers targeting 25-36 cSt for optimum efficiency. Always check the pump's minimum and maximum viscosity limits.

To pick a grade, estimate the steady-state oil temperature in the reservoir, then use the viscosity-temperature chart from the oil data sheet to see which grade lands in the target range. As a rough guide:

• Reservoir around 40-50°C: AW32 or AW46 often fits
• Reservoir around 50-65°C: AW46 is usually the better match
• Reservoir above 65°C: AW68 may be appropriate, but also consider improving cooling

3. Consider the climate

Ambient temperature affects both cold-start and running conditions. A machine in a cold-climate workshop may suit AW32, while the same machine in a hot, outdoor tropical environment may need AW46 or AW68. For equipment exposed to wide seasonal swings, a higher viscosity index oil (or a multigrade HV fluid) can help.

4. Consider pump type and load

Piston pumps are usually tolerant of a broader viscosity range, while vane pumps are often more sensitive. High-pressure systems benefit from a stronger film, while high-speed, low-load systems often favor a thinner oil.

Can you substitute one AW grade for another?

Mixing AW grades from the same base oil family is chemically safe, but it produces an in-between viscosity. Before moving from AW46 to AW68, confirm the pump's viscosity limits at cold start, flush if needed, and monitor pressure, temperature, and noise afterward.

AW32, AW46, and AW68 hydraulic oils differ mainly in viscosity, and that difference affects efficiency, wear protection, heat generation, and system response. AW32 suits cooler, lower-load applications, AW46 is the versatile standard for most systems, and AW68 handles heat and heavy duty. The best choice is always the grade that keeps your oil within the pump's recommended viscosity range at real operating temperature.

1. Is AW46 better than AW32?

Neither is universally better. AW46 is thicker and suits moderate-to-warm conditions, while AW32 is thinner and suits cooler conditions. The better choice is the one that keeps viscosity within the pump manufacturer's recommended range.

2. Can I use AW68 instead of AW46?

Sometimes, in hot climates or at high operating temperatures, if the pump's viscosity limits allow it. In cooler conditions, AW68 can cause sluggish response, cavitation, and extra heat. Confirm with the OEM before switching.

3. What does the number in AW32, AW46, and AW68 stand for?

It is the ISO viscosity grade, meaning the oil's kinematic viscosity in cSt at 40°C. AW32 is about 32 cSt, AW46 about 46 cSt, and AW68 about 68 cSt.

4. Does AW hydraulic oil have different additives depending on the grade?

The additive chemistry is typically similar across grades from the same product line. The main difference is the base oil viscosity, though formulations vary between brands, so check the data sheet.

5. What happens if I use the wrong viscosity grade?

Oil that is too thin can cause increased wear, internal leakage, and lower efficiency. Oil that is too thick can cause slow response, cavitation, higher energy use, and excess heat. Both can shorten component life.