Last three seasons I used Repsoil and genuine Suzuki engine oil and recently switched to Spectro4. That switch markedly improved the gear switching. Now they are much more accurate and "crisp". Maybe it is my bike. I was just surprised that genuine Suzuki motorcycle oil was not apparently the best for our gearbox.
http://www.spectro-oils.com/products/4stroke/spectro4.asp (http://www.spectro-oils.com/products/4stroke/spectro4.asp) and I got a 5 quart can for about $25 at a local dealership.
Granted I don't have as much experience with engine oil as many on this board but I decided to pass this finding along anyway.