IIRC, you CAN use GL-5 rated oils in the tranny AS LONG AS IT'S ALSO MT-1 RATED. I have a bottle of Valvoline that's GL-4/GL-5 & MT-1. That's okay to use in the tranny and diff.
75w-90 Mobil 1 synthetic gear oil is rated ONLY GL-5. That shit will eat synchros if it gets hot enough.
Ideally, for optimum (by a small margin) performance, you should use a GL-4 only rated oil in your tranny, like MT-90. It's nothing to lose sleep over, using a good MT-1 rated oil won't be noticibly worse than a GL-4 rated oil.
If you want the absolute BEST shit, check out this website:
http://specialtyformulations.netfirms.com/
They forumulate their oils from Group IV and V base oils, not Group III. Group IV and V are TRUE fully synthetic basestocks. From what i've read on BITOG forums (
http://theoildrop.server101.com/cgi/ultimatebb.cgi), this oil the the
shit.