My experience with both the older 3 speed and 4 speed transmissions whether Muncie or Borg Warner dictate that the use of straight SAE 90 weight gear oil is required for these transmissions to operate correctly. The straight 90 weight mineral based gear oil has the proper amount ash content in the lubricant for the synchronizers to engage without slipping. Most of the modern day gear oils are too slick which will cause synchronizers to slip in these older transmission causing gear clash and the need to double clutch the transmission in order for the synchronizers to engage. Slipping synchronizers create excessive wear to both the synchronizers and drive gears. I was able to purchase straight SAE 90 weight gear oil at NAPA or most farm supply stores.

Some of our members don't agree with this analysis, but the Chevrolet/GMC shop manuals state the use of this gear oil. GM engineers must have had a reason to specify this gear lubricant.

Hope this helps.

Tim

"Pay attention to the details! It ALWAYS pays off."

1949 Chevrolet 3100 Series 1/2 ton Pickup
1964 Chevrolet C10 (Ol' Yella) (SOLD)
1958 Chevrolet Biscayne 2 door (SOLD)
1970 VW Beetle