gear lube, preferably synthetic
Ignore all the crap even from GM (not that GM has ever been confused about anything, or told even the slightest untruth, or anything like that

) about this year uses this, for one year they changed to that, etc... the real deal is, the non-WC T-5s have brass synchros and should use gear lube; the WC ones have composition synchros and should use ATF. Although, lots of people (including myself) use gear lube in the WC models for its superior bearing protection without any problems.