I have been using the 75W90 diff oil since my 89 Z31 turbo and now on my 96 TT, no problems at all. For the 89 I did have to put in the additive. I put in the additive for the TT just to find out from this forum that it's not needed. No problems at all. For the tranny, I also have been using the MT90 oil. No issues at all. BTW, I live on the eastern shore of MD where it stays practically mild in the winters.