2004 Civic EX... Had a tranny rebuild. I complained that shifting into reverse had been hard at times. The syncros were fine. He put in Mobil 1 75-90 synthetic and told me it should make the shifting in reverse better. Was that a good idea?

 

Not sure if the Civic tranny has the same issues as my TL's tranny, but in the Accord and TL 6MT community, the go to MTF is ACDelco Synchromesh Friction Modified. The previous owner of my TL sold the car because he was sure the transmission was failing, I put in the Synchromesh and all symptoms of hard shifting and gear grinding disappeared; that was 45,000 miles ago.