3600Chevy,
I rebuilt my sons T-5 in his S-10 pickup. I had the same question. There are two different transmissions, World Class (WC) and Non-World Class (NWC). They have different synchro sleeves I believe. One has brass and the other Kevlar.
According to the assembly manual, the NWC uses gear lube and the WC uses ATF. These lubricants are compatible with the synchro sleeve material.
On the other hand, if you read all the dope on the 'net', almost everyone uses ATF as the heavier gear lube causes the transmission to be stiff and difficult to shift, especially when cold.
ATF has one of the highest viscosity index ratings of any oil made, so whatever box you have, this oil would certainly not lead to its destruction.
Lots of folks on this site have installed this transmission in their trucks, so I'll bet you get some hands on advice in short order.
Stuart
