So, I'd recomend ATF, that's what it's supposed to have.
I found 30w(I think) oil in my '93 when I bought it... probably to quiet down a noisy bearing. It worked fine cold(a little stiff perhaps), quieted it down... get it hot and it makes noise.
I decided to try gear oil... cold, it barely wanted to shift, wanted to grind etc. Once warmed up, it was great.
Swapped with Amsoil ATF, and while it was a little noisier, but shifts much better.
edit:
I will point out, just for reference, that I have a pretty worn transmission I picked up from the local wrecking yard for $170-ish. Turns out that what fluid came out of it was gray and full of little metal bits.
I'm currently running 4 quarts of Lucas Universal Transmission/Hydraulic fluid(close to 5w30 in thickness) and 1 quart of ATF in there, hoping to have something a /little/ thicker than straight ATF considering all the wear. It is working fine, but it's definitely got some bearing noises. Shifts fine cold, which is important.
I'm not going to /recommend/ it, though.