I have a 68 Plymouth Service Manual. "Use SAE 140 gear oil. If shift effort becomes too hard in cold weather change to SAE 80 or SAE 90 gear oil, ATF (Dextron) may also be used in extremely cold climates." I know that later 4 speeds came with ATF installed. Any 4 speed I touched had SAE 90 gear lube when it left. You can measure more wear in the countershaft, the input tip of the main shaft, the shifter forks where they interface with the synchros, ball bearings, and and the inner synchronizer drums if used with ATF over a long time compared to SAE 90 gear lube.
