What Lubricant is Used in The Final Drive (Differential)?

[Otto Skorzeny]

Interesting. So Bob the Oil Baron got a reply from GM that the recommended fluid to replace the original Oldsmobile fluid is SAE 90, multi-purpose gear lube. No mention of a multi-viscosity requirement or whether Dino. oil or synthetic oil is preferred.

Not a rumor, Schukey, whale oil was used as a friction modifier in ATF and other industrial oils into the 1970s. Old timer transmission guys claimed for years afterward that no automatic transmission ever shifted right again.

That was probably true to some extent especially for transmissions like the Hydramatic that were designed in the 30s and refined in the 50s. As technology caught up with the requirements of transmissions the problems diminished.

Modern transmissions were then designed around the newer, whale oil free, lubricants and didn't suffer their predecessors' fates.

[Schurkey]

Amsoil e-mailed me to say that their oldest product guide is from 1988, and that 1988 guide suggests their product "AGL" for the '66-67 differential.

AGL is 80W-90 gear lube, synthetic but not their "top line" synthetic.

http://www.amsoil.com/shop/by-product/g ... gear-lube/

[Otto Skorzeny]

Thanks Shurkey.

Tuesday I greased the front end and check the diff. I was not able to find straight 90 gear oil so I purchased Peak 80-90 GL-5 rated lube. It seemed to be the closest to the GM recommendation.

It was also Dino rather than synthetic since I figured whatever was in there was probably not synthetic anyway. I suppose Oldsmobile's original admonishment was meant to keep people from using too high viscosity oils like the 75-140 or so.

It was a pint low so I'm glad I checked it.