Hydraulic Oil 32 vs 46: Which Grade Does Your Equipment Need?

Over the past three decades working with hydraulic systems from agricultural machinery and construction equipment to industrial presses and marine applications I've seen the same question come up again and again: should I be running AW32 or AW46?

It sounds like a minor detail, doesn't it? WRONG!!! Use the inappropriate grade and you're looking at sluggish performance in cold weather, overheating in summer, accelerated component wear, and in worst cases, catastrophic pump failure. I've seen a mismatched oil grade cause £15,000 worth of damage to an excavator's hydraulic pump. The oil itself cost £40.

So, let's get into it properly.

Hydraulic Oil 32 vs 46 - Skip to the quick decision guide

⚠️ Important

This article is based on my personal experience and established industry best practices. It is intended for general informational purposes only. Always consult and follow the manufacturer’s manual for equipment‑specific requirements, safety instructions, and approved hydraulic oil specifications.

What Do the Numbers Actually Mean?

The numbers 32 and 46 refer to the oil's ISO viscosity grade (ISO VG) — specifically, its kinematic viscosity measured in centistokes (cSt) at 40°C. In plain terms, it's a measure of how thick or thin the oil is at operating temperature.

A higher number means a thicker, more viscous oil. A lower number means a thinner, more free-flowing oil.

Both AW32 and AW46 are anti-wear (AW) mineral hydraulic oils. The "AW" tells you they contain anti-wear additives to protect pump components and metal surfaces under pressure. The number tells you the viscosity. That's really the whole story but the implications of that difference are significant.

 Key Differences: Hydraulic Oil 32 vs 46

When to Use Hydraulic Oil 32 (AW32)

AW32 is a thinner oil. Because it flows more freely, it pressurises faster when the system starts up. This is critical in cold weather when thick oil can starve a pump of lubrication in those first crucial seconds of operation.

I always recommend AW32 for:

• Indoor equipment like machine tools, industrial presses and factory automation where ambient temperatures are controlled, and the design of the machine can keep the oil temperature regulated.

• Forklifts and pallet trucks operating in cold stores or refrigerated warehouses, however there are also special fluids specifically designed for ultra-low-temperatures. 

• Hydraulic rams and cylinders on lighter equipment with lower operating pressures

• Cold climate outdoor equipment operating through winter months in the UK, Northern Europe, or Canada

• Smaller, lower-pressure systems where the pump manufacturer specifies a lower viscosity range

  
  

The key thing to remember with AW32 is that it offers less film thickness at high temperatures. If you're running a system that generates significant heat under sustained load, for eg. a large excavator arm working hard all day, AW32 will thin out too much and you lose the protective oil film between moving metal surfaces. That's when wear accelerates.

💡 Expert Tip

If your equipment operates in temperatures regularly below 0°C and your OEM allows it, you might even consider a premium AW32 with a higher viscosity index (VI). High-VI oils maintain viscosity more consistently across a wider temperature range. It’s worth the extra cost on critical equipment.

When to Use Hydraulic Oil 46 (AW46)

AW46 is the most widely used hydraulic oil in the world — and for good reason. It's the go-to grade for most of the mobile plant equipment, and it's what most OEM manuals will specify for general-purpose outdoor use in temperate to warm climates.

Reach for AW46 when you're running:

• Excavators, diggers, and tracked plant — the most common application I see day to day

• Dump trucks and tippers with high-cycle hydraulic tipping systems

• Agricultural equipment — tractors, combine harvesters, telehandlers — operating through spring and summer

• Log splitters, woodchippers, and other high-pressure outdoor equipment

• Any system operating in ambient temperatures above 10°C on a consistent basis

The thicker viscosity of AW46 maintains a stronger oil film at operating temperature, which translates directly to better pump and valve protection under sustained high-pressure cycles. On a busy construction site in summer, this matters enormously.

Hydraulic Oil 32 vs 46 - The Quick Decision Guide

Can You Mix Hydraulic Oil 32 and 46?

This is probably the question I get asked most often, and the short answer is: technically yes, but you really shouldn't make a habit of it.

Both AW32 and AW46 are mineral-based anti-wear hydraulic oils with compatible base stocks. Mixing them won't cause a violent chemical reaction or immediately destroy your equipment. What you'll end up with is a blend that sits somewhere between the two viscosities — roughly ISO VG 38-40 depending on proportions.

The problem is that you've now got an unknown, unspecified fluid in your system. The additive packages in different brands of hydraulic oil aren't always identical. Mixing can cause additive interactions that affect anti-wear performance, demulsibility (how well the oil sheds water), and foam inhibition. Over time, that's not where you want to be.

⚠️ Important

Never mix mineral hydraulic oil with synthetic hydraulic oil, or with biodegradable (vegetable-based) hydraulic fluid. These are genuinely incompatible and can cause seal damage, sludge formation, and rapid oil degradation. If you're switching fluid types, flush the system properly first. If you're topped up in an emergency with the wrong grade — not ideal, but not a disaster. Get a proper drain and refill done at the next service interval, and you should be fine.

What Happens If You Use the Wrong Grade?

In over 20 years, I've seen the consequences of wrong-grade oil play out in several ways. Here's what happens:

Using AW46 in a Cold Climate (When AW32 Is Needed)

Cold, thick AW46 is sluggish. On start-up, it takes longer to circulate through the system and in those few seconds before the pump is properly lubricated, you're generating accelerated wear. You'll also notice slower response from your hydraulic controls, increased pump noise, and in severe cases, cavitation where the pump is essentially trying to draw in oil faster than it can flow. Cavitation destroys pumps. I've seen it reduce a £3,000 hydraulic pump to scrap in under 200 hours.

Using AW32 in a Hot, Heavy-Duty Application (When AW46 Is Needed)

As the system heats up, AW32 thins out beyond its effective operating range. The oil film between metal surfaces becomes too thin to prevent contact, and metal-to-metal wear begins. You'll see increased operating temperatures, possible overheating, higher system pressures as internal leakage increases across worn components and eventually pump and valve failure. The degradation is gradual, which makes it dangerous because operators often don't notice until significant damage is already done.

Final Thoughts

The difference between AW32 and AW46 comes down to one thing: matching viscosity to your operating temperature and load. Get that right, and your hydraulic system will run efficiently, your components will last, and you won't be paying for avoidable repairs.

As a rule of thumb: cold conditions, lighter duty, indoor applications — reach for AW32. Warmer conditions, heavy plant, high-cycle outdoor work — AW46 is your friend.

When in doubt, pull out the OEM manual. It's there for a reason, and the engineers who designed your equipment tested it thoroughly. There's rarely a need to second-guess a clearly specified fluid grade.

And if you're ever in a situation where you genuinely don't know — take a small oil sample and send it for analysis. A fluid analysis from a reputable lab costs around £30–50 and can tell you more about the health of your hydraulic system than any visual inspection. It's one of the best investments in preventive maintenance you can make.

Lister Fluid Power provides hydraulic oil sampling and analysis free of charge as part of our ongoing commitment to sustainability. By helping reduce contamination, we minimise unnecessary oil waste and support cleaner, more efficient hydraulic systems. This complimentary service is available to all our customers across the West Midlands and South Wales.