Originally Posted by R_McAnulty
So picking up this thread. I’ll be going with 90 w mineral oil for the tranny.
Question, is the same oil fine to top up the rear end ? It’s a little leaky leaky ……

Thanks to all……..
Depends on the rating of the mineral oil. These trucks ran on rear axle gear oil from before GL-5 was developed. If it's straight Mineral oil without a rating like GL-4, then the service manual says
"CAUTION: Straight Mineral Oil gear lubricants must not be used in Hypoid rear axles or two-speed rear axles.

Save your mineral oil for the tranny. It's probably less expensive to buy the prevalent GL-5 gear oil to top off your rear axle.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.