[bjc]

I think f5 may have addressed this question at some point, but I'll jump in here since I know how hard it is to get help with 5-speed questions.

I seem to remember that the manual tranny doesn't use synthetic. Assuming that's correct, I'm not inclined to move away from the OEM fluid in favor of a "superior lubricant" (read - synthetic) because I'm already happy with how my truck shifts.

I know a few people (non-Hummer owners) that switched to synthetic with mixed results. Those that switched in the winter found their shifting improved, but it was worse in the summer when the fluid became too slick for the syncronizers.