Interesting question in view of the bearing and 2nd gear issues with the 6 speed manual gearbox. The bearings that fail are sealed ,so the transaxle oil choice should be irrelevant- unless one oil provides better cooling than another ? The 2nd gear teeth are just too small -so oil spec won't help?
Here is an interesting discussion of GL4 vs GL5
GL5 gear oil bad in tranny,fact or myth? | ATF, Differential, Trans, Brakes, P/S | Bob Is The Oil Guy