Differential fluid weight

Just wanting to confirm with any racers out there…

About a year ago, i changed my differential oil from 90W to 140W.

I have a B&M shift kit for firmer shifts.

Every once in a while, i test out flooring it from 0-60. If I keep it floored, when TH400 shifts from 1st to 2nd gear, it’ll chirp the tires and carry on straight. But since I’ve switched to 140W oil, I’ve noticed if i do this within first 20 minutes of driving, instead, the car will drift left about a half foot before the limited slip corrects.

Is this just a problem with getting that thick oil up to temp?

Did the new oil have the correct additive in it being LSD?

This has been in my garage for the last 30 years.

Why did you feel that the diff need heavier oil Greg?

My logic says that when cold, the 140W is too thick and slippery and thus the PSD slips.
Also you add more overall friction with the thicker oil.

And indeed, did you put LSD additive?

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After only one cup of coffee, I think that if you had an open diff, at the first WOT shift, one wheel would lose traction, but the car would steer straight. With your situation, I think both rear wheels lose traction for a moment, and so the rear changes direction briefly. . As the oil warms up the situation becomes less aggressive.
Now for more coffee.


I agree with Dave.

What you are feeling on the #2 shift sounds like an LSD behaving like an LSD.
What you are feeling on the #1 shift sounds like an LSD behaving like an open diff.

I’m really not sure what role the 140w oil plays in this


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Yes, it is LSD gear oil.

The 140W was recommended for heavy-duty. My guess is just to let the oil heat up with some driving before trying anything heavy duty? Seems to take 30 minutes.

Just wondered if anybody who also runs 140W can concur.

I like this stuff, but then my car is used for cruising-duty. :slightly_smiling_face:

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110W, that’s a good compromise between the two weights. Maybe I’ll try that next change. I really like Redline products. Is that Limited Slip, or do you need to add?

No whales will be harmed during this refill. Nothing to add.

Perhaps @Doug_Dwyer can corroborate, but I think that 140W would only be called for in sub-Saharan Africa.

That doesn’t make sense.

And here is why I changed to 140W gear oil, some advice from Dick Maury 2 years ago:

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It makes perfect sense, in that context: just how hot do you think you’re getting that differential?

My bet is not that hot.

@gregma are you running straight 140W? Or a multi weight like 85W-140?

Multi weight 85/140W. Which is essentially a 140W.

Also, its not ambient heat that determines gear oil viscosity. LOL

I am quite aware of that: again, and this is a general comment, the higher the temperature that a given device works at, especially one of the age of the our cars, the higher the viscosity oil needed.

Unless you are running massive amounts of horsepower in a track situation, I seriously doubt that your differential needs 140W on the street.


I always wondered who used the 140w stuff. I figured it was for tractors or irrigation pumps or something! Shows what I know, eh? :slight_smile:

In anything but track use or trailer towing I’ve never considered heat to be an issue with diff oil. I change mine every 25k miles or so, FWIW.

I’m loathe to dismiss any recommendation from Dick Maury, though. If anyone knows Jag diffs, he does.


Not doubting Dick’s recommendation: in fact, what he also stated was the important bit is to ensure the lube is rated as compatible for hypoid gears, not so much the weight.

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Thanks for your opinions, I will give them some thought when I next replace the gear oil.

FWIW, I used ATF in my race cars’ diffs: in FV, I wanted minimal drag.

I’d not necessarily recommend such for a Jag’s diff, but it was effective.

Funny enough, my 1990 Mercedes 190E manual also asks for ATF for the gearbox.