M90 75w90 gl-4 for diff fluid?
 

Ok,so I changed out my tranny fluid and diff fluid and put in the proper tranny fluid,but looks like the wrong diff fluid! Is it ok to run gl-4 75w90 in the diff or should it be changed right away? I just did my oil cooler install and oil change as well so I was thinking to change the diff fluid to gl-5 on my next oil change. Think I might run into any problems before then? I put in redline.

You'll be fine for a while. GL4 has a different set of specifications designed to provide enough friction for synchronizers in a typical manual transmission to work properly. If you're after the least amount of friction as far as gear lube is concerned, try GL6.

Right on man,thanks. I didn't figure it would hurt it and I will be changing out at next oil change. I got 45,000 kms on the car and Judging by the crap coming out,it'll be beneficial to change it again soon. As for the gl-6,I read up that they don't make it anymore. Unless they call it something else now?

No idea, the standards on gear oil have been used for decades and they work. I can't see they were changed. There are GL6-spec gear oils out there, they shouldn't be difficult to find.