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ISO 46 Hydraulic Fluid vs 10W-30 Motor Oil in Hydraulics What’s Actually Better

H

HeavyCons

New member
Hey guys,

I’m a bit confused about hydraulic oil choices and I want to make sure I’m not chasing the wrong problem.

I’ve got an older hydrostatic skid steer that has always used 10W-30 motor oil in the hydraulic/hydrostatic system (that’s what the manual calls for). The machine runs and works, but it makes a pretty loud squealing noise when I’m turning and sometimes when I’m using the hydraulics under load. That got me wondering if I should switch to iso 46 hydraulic fluid (AW-46 style) instead. In my head ISO 46 sounds like “proper hydraulic oil,” but I’m not sure if it would actually help or if it could make things worse.

For reference, here’s a typical ISO 46 hydraulic oil datasheet showing it’s 46 cSt at 40°C: PDF

1-)In real world skid steer hydrostatics, is ISO 46 a smart switch from 10W-30?

2-)Could oil choice actually reduce squeal, or is squeal usually a different issue?

3-)If I do switch, what’s the right way to change over (filters, flushing, etc.)?

I’m in a warm climate most of the year, so cold-start viscosity isn’t a huge concern.
 
You’re asking the right questions, because the internet usually turns this into “oil wars” when the real answer is: oil choice matters, but it doesn’t change physics, and it usually doesn’t fix a mechanical or control problem by itself.

First, separate the symptoms:

Squeal while turning on a hydrostatic machine is often related to the drive/hydrostatic loop (pumps, motors, charge circuit, reliefs, swashplate control, linkage adjustment), while “aux hydraulics feel weak” is more related to the implement/aux pump and valves. On many machines those are separate pumps, so chasing “ISO 46 vs 10W-30” for a turning squeal can be a red herring. That separation is exactly what a knowledgeable user pointed out in a similar discussion: drive pumps and implement pumps can be different circuits and the noise vs flow complaints can be two separate problems.

Now about ISO 46 vs 10W-30:

ISO 46 is a hydraulic viscosity grade measured at 40°C (104°F). A typical ISO 46 hydraulic oil will list “46 cSt at 40°C” on its datasheet.
10W-30 is an engine oil grade and the numbering system is different. That’s why direct “ISO 46 = thicker than 10W-30” statements are often wrong or only true at specific temps. In the competitor thread, one of the best points was that viscosity comparisons across ISO and SAE aren’t apples-to-apples and at operating temperature they can be quite similar.

So what happens if you switch?

In cold weather, ISO 46 can behave “thicker” than some engine oil choices at startup and can change noise characteristics (sometimes quieter, sometimes more sluggish).

In hot operating conditions, 10W-30 can end up behaving effectively like a “30 weight” at temp, and depending on exact temps it may be comparable or even “thicker” than some ISO 46 fluids in the real circuit. That’s why changing to ISO 46 won’t magically “increase flow” or cure all noise.

If your machine is rated “X GPM,” the oil type does not raise the pump’s displacement. Flow is mainly pump size × engine RPM. Oil viscosity can change leakage and efficiency, but it won’t turn a 9 GPM machine into a 15 GPM machine. (Where you do “feel” improvement sometimes is reduced internal leakage if the system is worn and the oil is too thin for the conditions.)

Will oil choice reduce squeal? Sometimes, but treat it as a clue, not a cure.
Thicker oil can change the sound of hydrostatics, and hydrostatics can be naturally noisy. Especially on high-traction surfaces. That point was also made in the competitor discussion: hydrostatics vary in how loud they sound even when working fine, and heavier oil can sometimes quiet them.

If you want a practical checklist before changing fluids:

-Check hydraulic oil level (obvious, but low oil = aeration noise)

-Inspect suction side for air leaks (cracked hoses, loose clamps, hardened O-rings)

-Replace filters, especially charge/suction-related filters if applicable

-Verify linkage/control neutral adjustment (badly adjusted linkage can create “pump is screaming” symptoms even if the pump isn’t the root cause)

-Make sure the correct oil spec is actually being met (some OEM “10W-30 hydraulic/transmission” fluids are not the same as random engine oil)

If after all that you still want to switch, ISO 46 hydraulic oil is a reasonable choice in many hydraulic systems, but: follow the manufacturer spec first. Some machines that call for 10W-30 are doing it because they want certain shear stability/additive behavior across temperature ranges, not because “hydraulic oil is bad.”
 
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Let me explain ISO 46 in plain language because once that clicks, the rest gets easier.

When people say iso 46 hydraulic fluid, they usually mean an anti-wear hydraulic oil in the ISO VG 46 viscosity grade (you’ll see names like AW-46 or HM 46 depending on region and spec language). The key number is the viscosity at 40°C. A typical sheet will literally list “Kinematic Viscosity @40°C: 46 cSt.”

So ISO 46 is not a brand. It’s a viscosity “bucket.”

Now, why do people use 10W-30 in hydraulics at all?
Because some OEMs spec multi-purpose fluids that behave well across a wide range, and some equipment uses a common reservoir for hydraulic + hydrostatic + sometimes even drive components. For example, John Deere Hy-Gard is marketed as a hydraulic/transmission oil and documents show it sits between ISO 46 and ISO 68 depending on temperature (and it’s often described alongside “10W-30” language). That’s why “10W-30” on a skid steer manual doesn’t always mean “any cheap engine oil.” It might mean a fluid designed to meet certain hydraulic/transmission requirements while falling into a familiar SAE label.

So what should you do if you’re warm climate?
Warm climate pushes you away from “super thin” fluids, but you still want what the system is designed for. ISO 46 is often a good general-purpose hydraulic viscosity for moderate climates, but OEMs might want different viscosity or additive package for hydrostatics.

If the real goal is squeal reduction, here’s what I would do as a practical owner:

  • *Confirm that the noise is truly “hydraulic squeal” and not belt, chain, brake chatter, or tire scrub.

  • *Check whether squeal happens more on concrete/high-traction surfaces (hydrostatic systems can sound louder there).

  • *Change filters first and inspect suction plumbing for any chance of aeration.

If you still suspect oil: choose a fluid that clearly matches a known spec, not “mystery 10W-30.”

If you decide on ISO 46, pick a known AW/HM ISO 46 product with a real PDS/SDS. Example SDS/PDS documents consistently show ISO 46 fluids at 46 cSt @40°C, and you can compare viscosity index, pour point, etc.
 
ISO 46 is a viscosity grade, not a magic fluid. If you want to switch, pick a quality AW/HM ISO 46 with a real datasheet like this one and stick to one product family
Link
 
This makes a lot more sense now. I was definitely mixing up the idea of “hydraulic oil” with “hydraulic problem.” I’m going to start with filters and a suction-side inspection first, then decide if a fluid change is even needed. If I do switch, I’ll document what I used and whether it changed the noise at all.
 
iso 46 hydraulic fluid comes up a lot because it’s a common industrial grade, but many skid steers (especially older hydrostatic setups) were spec’d around multi purpose fluids. If you post your machine model and climate, members can point you toward the most realistic viscosity choice.
 
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