gear oil

Belray Gearsaver is the way to go. Get 75W or 80W. Kennedy Powersports reccomends ATF, but I've never tried it. He claims it allows for higher horsepower.
 
i changed the trans oil today and through some valvoline mercon v in it and it shifts great plus no slipping!! this is compared to using maxima mtl 80w.
 
During summer time i would use thick 80w
During Winter i'd try using something a little less thinner.
You can use something thick, but you have to let the bike warm up first more than usual. I use Bel-Ray 75w cold temp and 80w in summer time. Lol Kinda a joke. I have a warm garage though.