Pretty easy to tell if it's oil or gas, gas will evaporate, oil will remain after a few hours.
I'd check if you have a cyl that's not firing. Pull the plugs and look for a wet one, run the bike for a few minutes and then pull the plugs and look for fuel.
You can attach a timing light to each of the for spark plug wires, one at a time and make sure each one is firing.
A leak down test tells a lot about how it's sealing, not very expensive from Harbor Freight.
I'd think the valves have to off quite a bit for it to pass oil, and passing unburnt gas out the exhaust seems unlikely because it would have to be quite a bit or it would just evaporate.
Maybe a bad oil control ring, the pistons have 3 rings and the oil control ring scrapes the oil off the sides, if they lock up, oil could be getting into the chamber and out the exhaust.
Also, valve stem seals. Had it happen to a car, the valve stem seals crack.
If the valve stem seals are bad, you can replace them without removing the head. I can explain how if you want.
If it's the rings, you'll need to drop the pan and remove the head.