90W axle lubricant

[Matt Harwood]

I'm going through all the fluids in the '41 Limited and the manual recommends 90W SAE EP oil for both the transmission and the rear end. Is there a modern equivalent? At the auto parts store today, I saw several multi-viscosity blends, but no straight 90 weight and most of the rear end lubricants had limited slip additive already in them. I presume the EP is extreme pressure for hypoid style rear ends, and I think any GL4 or GL5 rated oil would be safe with the original yellow metal bushings.

What are you guys running in the axles? 85W120?

Edited April 18, 2016 by Matt Harwood (see edit history)

[Daves1940Buick56S]

You are good with GL-4, GL-5 may not be safe for yellow metal. If you axle is worn at all, try 85W-140, otherwise 85W-90 should be OK.

 

Cheers, Dave

[Matt Harwood]

Thanks, Dave. Picked up some 85W90 GL-4 at NAPA this morning. That's probably good for transmission and rear end, since both call for 90W.

 

Interestingly enough, in doing some reading about this subject, it appears that these are not multi-weight oils in the traditional sense. Instead, they are all 90 weight oils that fit in a window of viscosity that can be called 90W. That is to say, a 75W90 is on the thinner end of what is permissible for a 90W while 85W90 is on the thicker side. They don't actually have polymers in them or viscosity modifiers like multi-weight motor oil. So anything with 90W is probably 90 weight oil, but the prefix number will tell you if it's thicker or thinner. Not that it matters too much for these old hulks, but kind of interesting to know.

 

I also learned that GL-5 is probably deadly for the synchros in our old cars, so it should be avoided. There are plenty that are marked GL-4/GL-5 but only because the GL-5 rating supplants the GL-4 in their testing procedures. So the guy will tell you that GL-5 is what you should use because it's technically backwards-compatible, but real GL-4 is safe for old cars whereas GL-5 is not.