you need a 75w-90 GL-4 lubricant. the more commone GL-5 is corrosive to the soft metals inside the transmission(like synchros).
I would reccomend "Redline MT-90". its a full synthetic 75W-90 GL-4 lubricant. i've have extremly good experience with it, primarily with cold weather shiftability. it also makes it a little easier to powershift, but just a little.
Its supposed to have an additive package that makes it more ideal for the friction requirements and load requirements of a manual transmission.
that's the word on the street. its meant for differentials, which have no soft metals to worry about. there was a discussion on tranny oils a while back where everyone was saying gl-5's at least USED to be bad...but they were't sure if they still are.
well, i originally had a GL-5 lubricant in my tranny, and when i drained it out, the oil had a green tint to it(like tarnished brass). so i feel fairly safe in concluding from experience that gl-5 does indeed corrode soft metals.
so i would recomend that you replaced it with a gl-4 lubricant, lest your synchros retire.