I can't remember the numbers off the top of my head...but I believe our gearbox's need GL4 75w 90 gear oil...I'm saying this because the Mobil 1 gear oil at most parts stores is GL5...not 4...and GL5 has sulfur additives and other things that are not good for the type of syncro's our transaxles have...I had a probe GT before my P5 (which had a similar Taxle), installed Mobil 1 gear oil...had no idea what any of that GL stuff meant...and ended up losing 5th gear just a week later...I'm sure it was close to going anyway, so it might have been coincidence...but after some research I found out about the different GL ratings...
Royal Purple and Red Line (as well as many others) do make GL4 gear oils...I don't know if Mobil 1 does or not, I just know every time I see it I check the back and its GL5...
I have had Red Line MTX-90 or whatever it was in mine for a while, and its been great...speaking of which though I should change it, its been 75k miles or so...