well maybe I am wrong here, but IMO< oil burning is more a blue ish smoke
WHITE< tends to be coolant burning off >
BUT I doubt this atv is a liquid cooled motor, so that rules out the coolant side of things LOL
so, then next for WHITE smoke, to me is water!
do you ride in deep water often?
if so, maybe water is pooling up in your exhaust! and then burning off when riding and hot! and lots of throttle just blows the water out closer to tail end and you see it burning off?
your spark plug will tell you if your burning oil, as well, a lower level on your dip stick, so, good to check both of them and see what there telling you!
IF atv was ever sunk, that opens a whole other reason your motor is smoking