Don't laugh, but I like to use
Pro Honda HP Transmission Oil - 80W/85W , I've used this stuff in everything 2 stroke and 4 stroke that i've owned as far as quads/dirt bikes go. Its designed for 2 stroke's but came highly recommended by the honda dealer for the 2004 Honda crf450RR I owned, been using it ever since.
Heres a link to check it out. I:I
Pro Honda HP Transmission Oil | ATV | Rocky Mountain ATV/MC
Honda's claim to fame with this oil
" HP Trans Oil offers ultra-high film strength and shear stability to minimize transmission wear, while its high thermal stability prevents foaming and premature oil breakdown.
Special anti-friction additives allow smoother shifting, while minimizing drag and power loss. Anti-shear additives increase clutch life, reducing clutch slippage. "
There are many choices out there for oil, just make sure you pick a decent one. I would steer clear of motor oil being used in the tranny not because its harmful, but because it doesnt offer the same protection of other oils that are available to you.
*Disclaimer*
This is just my .02 and im new here, so take my advice lightly and do some research on any products I may have influenced you to use.