I initially went with 90W gear oil when I first swapped a T5 in my '81 because that's what I always thought went in a manual transmission. It worked okay, but shifting was not the best.
When I went to change the oil, I read in a manual for an '84 that came with a T5 that the recommended fluid is ATF.
I tried it, and shifting improved considerably.
1981 CJ7, GM 2.5L (Iron Duke), T5/D300, D30/AMC20 w/one-piece axles, 4.56 gears, 3" Black Diamond springs w/RS9000 shocks and 33"x10.5" BFG AT's on black 15"x8" American Racing wheels