I changed out my transmission fluid in my 2003 civic si. After driving around for about 20 min. I noticed that third gear was grinding as I shifted. If I baby it, it doesn't grind. I used Royal Purple Syncromax to refill my transmission. Any info as to why my transmission is grinding would be helpful.

 

Remove the transaxle and inspect/repair the 3rd synchronizer assembly.

 

ouch. kinda one of those don't fix what isn't broken curses.

 

I had this happen before. The trans is not getting along with the new fluid. Running that RP fluid is not a bad thing but the trans is in need of an overhaul. They don't last forever.

 

Royal purple tends to do that for whatever reason. Drain it and refill with Torco MTF or Torco MTF see what happens.

That said it probably needs a 3rd gear set.

 

I put torco mtf in one of my old series previously running oem. It started grinding third at almost any rpm vs rarely before at high rpm. Tired synchros dont like new slippery fluid.