trans fluid survey

An MTL 75w-90 GL4 is good for heavy duty/hot operation. The newer GL5 hypoid gear oils in those lighter weights are probably OK, but make sure is a dual-rated GL4/5 rated fluid that is not going to eat the synchros. I'd stay away from anything 140 - too heavy. You won't notice it with the FMs in a synthetic variety, but just not needed - overkill.

ATF was the factory fill before modern synchromesh fluids came on the scene. Those are the best choices for all around use nowadays.