Never put gear oil in a bert! It will take the clutches out. I use Spectro Race Trans fluid in mine and haven't had any issues in years. You can get it from Behrents, Bicknell sells Torco trans fluid or you can use Amzoil trans fluid. I've used all of these without issues. Also, whichever type you choose it should be changed about every 6-8 races or at least twice a season.