I've read numerous threads of how your not supposed to use mobil 1 75w90 in the tranny because it eats the brass syncros? How true is this and is it something to be concerned about or should I not worry about it and replace my fluid with mobil 1 75w90.

 

use Redline MT-90 which strictly a API GL4 approved tranny fluid. What you read is true- Mobil and many other 75w 90's are either GL5 or both GL4 and GL5 approved. You really need the reccommended GL4 because GL5 has sulfur in it which can eat away at your brass syncros.

 

im running the redline in my car and oh how smooth it shifts =D

 

Only thing I ever use in my transmissions is Redline MT90 or Motul 300.

 

I've always run Mobil 1 in all of my transmissions and never had any issues. I do plan to try motul when I'm done breaking in my clutch though.

 

Old thread but... i would HIGHLY recommend not using mobil 1. I lost 3rd gear synchro about a week after the change, and first/reverse a while after. The drained fluid was so thick I felt like i could move it with a magnet.

Not sure if it's the cause, but it certainly didn't help. Make sure whatever goes into your transmission is GL-4 only.