Is it bad to use the differential fluid in the tranny gearbox? (ie Redline 75w90 high performance full synthetic gear oil) The label on the container says it may cause gear clash in the tranny. Should I be worrying about blowing some syncs or perhaps a whole tranny (on an 03 wrx)?

 

Yes, using a synthetic that is too slick will cause issues with your synchros. So far, Lightweight or Ultralightweight Shockproof seems to be the bext at protecting gears and bearings while helping synchros work.