Need help with gear oil selection

It has been a long time but thinking the guy that rebuilt my

18 spline HEMI 4 speed out of a 440 6 Pak Cuda that had every other tooth on the synchro removed ( was in drag car before I got it) told me to use automatic trans fluid

Drove my 340 Challenger on the street for 17 years yeah needed new synchro but was fun for many years

I did put regular gear lube in my Dart 4 speed

Guess it depends on what you are going to do with it