I have been doing some research but can't seem to figure out whats the best or oem brand Audi uses for the tranny or the diff. I believe its 75w-90 for tranny and 90w for diff. I have been reading Redline or Motul. I have 1.8t stock five speed. I was thinking royal purple but heard its not good for the synchros.

 

I would say go redline mt90 in the tranny because it is a gl4. Any fluid that is a gl5 isn't good for the brass syncros in the transmission. As for the diff any synthetic 75w90 should be ok. I used mobil 1.