Best way to find out what oil to run is to use the lightest oil you can without slipping the clutch.
Too heavy and the clutch gets grabby, it also reduces power.
It mostly depends on the state of your clutch and what temperature it is where you ride.
If it's usually hot outside, a heavier oil helps...but if it's cold or you run short distances (like drags or hillshooting) you won't heat up the oil very much, so ATF is good in that situation.
Absolutely nothing with friction modifiers, as they deposit on the clutch fibers and cause slippage too.
Other than that, replace the oil OFTEN.
The more you replace it, the longer your gears will last.
Wet clutch fibers erode and are entrained in the oil, meaning the same fibers that cause friction are now in the oil getting mashed inside the gears.
High dollar oils do not change this fact...so no matter how good the oil is for the gears themselves, the fibers will continue to accelerate wear if you don't change it often.
The cheapest oil you can find replaced often is better than the most expensive oil replaced every now and then.

There's already a few thread here about transmission oil, and you'll be able to find what everyone likes to run in them.

I read about gear oils some where else on here but think it was in a clutch convo that's why i made this form so you can find info about gear oils