I use Motul 75/90. Its like 30 bucks per litre but it absolutely kills the motor oil that was in there. It shifts smoother, clutches more consistenly, and makes a ton less gear noise.
If you can't get to a dealership then look around for some belray gear saver 75w through 85w 80w being ideal. But yamalube is good stuff too. Royal Purple is always available at any autozone and would be another good pick.