75W90 Gear Lube For Front Differential - With Or Without Friction Modifier

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Mar 17, 2011
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I am changing the front differential fluid in my 98 Expedition.
The spec calls for 75W90 Gear Lube.
The 2 brands I commonly use are Synthetic Valvoline and Mobil 1. Both have friction modifiers added.
Would that be a problem for the front differential?
I also saw a Lucas brand Trans and Diff Lube, that didn't say anything about friction modifiers, but did have additives.
"Lucas SAE 75W-90 Synthetic Gear Oil is a non-foaming, super slick, long lasting lubricant designed especially for heavy duty or high performance applications where other gear lubricants just aren't good enough. Contains a special additive package that cushions gears and resists "squeezing out" under extreme pressure situations where other gear lubricants just aren't good enough. Specially designed to stand up to high temperatures without losing its lubricity."
Recommendations? Everything else without additives or friction modifier seems to be a store brand
 
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Gear oils with added friction modifier shouldn't harm non-LSD diffs, although some gov locker type diffs may not necessarily like it, is this just a regular open diff or some type of LSD or gov-locker?
 

JHZR2

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No issues. I run lubes with FM in them win open diffs. At one point long ago I thought I had read that some components in the FM were actually good for the gear faces.
 
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All that talk is for traction control units in OEM rearends, not the simple diffs in the front.
 
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I was under the impression that a friction modifier might harm a diff that does not require it. But I simply do not know and am confused by it and why some need it and some don't. It's why I've held off doing about 3 different vehicles right now, b/c the topic is very confusing and figuring out the coded language used by manufactures is also way overly complicated and frustrating. The manuals are poorly written, you have to hunt for obscure codes on VIN numbers and axle pumpkins, then go cross reference against equally obscure tables and codes... it's a real headache.

I simply don't understand why they make this more complicated than it should be; well actually I do, it's a nefarious ploy to force you to go to a mechanic.
 
Joined
Dec 19, 2019
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I was under the impression that a friction modifier might harm a diff that does not require it. But I simply do not know and am confused by it and why some need it and some don't. It's why I've held off doing about 3 different vehicles right now, b/c the topic is very confusing and figuring out the coded language used by manufactures is also way overly complicated and frustrating. The manuals are poorly written, you have to hunt for obscure codes on VIN numbers and axle pumpkins, then go cross reference against equally obscure tables and codes... it's a real headache.

I simply don't understand why they make this more complicated than it should be; well actually I do, it's a nefarious ploy to force you to go to a mechanic.
Friction modifier should do no harm in a standard open diff, the only place where friction modifier may be bad is in locker diffs like the governor style lockers that GM liked to use, they're intended to lock up and have the clutches firmly engage when the governor meets the conditions to engage, you don't really want any slippage of the clutches which unnecessary friction modifiers could cause to happen.
 
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