I tried Redline 75W90 in the Diff, and 75W90NS in my tranny for ~15k km.
I dont have any hard data to show it was good or bad. Shifting was good, but by the time i drained the oil for a motor drop, The car had some 2nd gear issues. It didn't grind unless i upshifted after reving very high, it was just stuburn, especially while downshifting.
I double clutched the whole 15k km. And by the time I dropped the motor, double clutching was the only was to get it from 2nd to 1st in a reasonable amount of time (rather than holding pressure on the shifter waiting for it to pop in)
So, i'm giving Royal Purple a try in my ST205 tranny. Its hella expensive. 22$ retail/L, i got it for 12
The bottle says its both GL-4 and GL-5 certified.
I dont know if the degrading performace of the tranny was due to age or the oil. There was only 65k km on the tranny.
Royal Purple also makes Powersteering fluid.