The original fluid calls for 75/90W GL-5 gear gear lubricant, however, the standard stuff likes to eat the brass synchronizer rings and will eventually destroy the rings.
I've put in RedLine MT-90 before, as it's brass friendly stuff. Worked well, but it's expensive.
10W30 motor oil is the new MOPAR parts standard for the AX15 transmission fluid. I currently have it in my AX15 and it works fine. I change the transmission oil when I change the engine oil.
Some people use Pennzoil Synchromesh.