It's not necessary to "clean up" the transmission. You could try a different spec of gear oil in hopes of better synchro action, but chances are, they are just worn, and the only true cure will be tearing down the transmission and replacing the synchro rings.
Redline MT90 is probably what you used, and is pretty good stuff. There's also redline MTL, which is a bit thinner I think. I've had good luck with Motul gear lubes in terms of smoothing worn synchros, but again, if they're worn out, the only true cure might be replacement.
