FYI, ATF is the recommened oil for a Mopar 4 speed trans. The manual does say that gear oil can be used in hot climates but ATF is the first recommened oil.
Chuck,
I have heard of some guys using ATF in the winter up north but never in warm weather. The manual calls for SAE 140, but states that this can be replaced by SAE 90, SAE 80, or ATF "if shift effort becomes extremely high" in cold weather but that this should be "replaced by SAE 140 in warm weather" I have always used 75W-90 in mine. SAE 140 is actually the first recommended lube in the manual.