The Stovebolt Site Forums The Shop Area General Truck Talk 1950 GMC Manual transmission fluid

As the author of the article mentions, a lot of confusion stems from the fact that the current rating system for gear oils was not around when lubricant for these older transmissions was originally specified. Trying to find a GL-4 or GL-5 reference in 1950's service information will be an exercise in futility, as those specifications did not yet exist.

If your service information specifies SAE 90 gear oil, then I would probably:

1) Just use, as Mike B and others mention, the 90 weight tractor supply stuff. It seems like a number of folks on this site use it without issue. (Although, it would be interesting to see the data sheet for this product to see how it compares with other gear oils)

or

2) Look at the viscosity chart in his paper, and note the range for SAE 90 gear oil. Surprisingly, it is pretty large. However. as he says, most 80W-90 gear oils tend to fall in toward the lower end of the range but should still work fine.

The main point, is to avoid using a GL-5 rated oil in an older vehicle with a synchronized transmission.

Best Regards...