Heard a few different things so I will begin with the rumor I heard. Do NOT run synthetic oil as it messes up our paper blocker rings. They went to carbon blocker rings in 2001 so in those transmissions, I would run RP Syncromax or AMSOIL Universal ATF or AMSOIL Torque-Drive Synthetic Transmission Fluid...although I may have heard we can run Amsoil ATD or in our earlier transmissions as the base for the synthetic is different...lots of big words.
On second thought, just get the stuff from GM. Then you don't have to worry about it.