Once an engine gets to turning so fast, the oil doesn't have time to get "wiped" back into the pan..
& is actually forced on top of the piston.. & it burns like fuel and comes out the exhaust..
Your problem lays either within the inj. pump/governor{stuck rack/broken gov. spring} or with the engine itself.. worn out rings, picking up/allowing oil on top of pistons.
OR could be something as simple as a clogged blow-by tube.. reburning the engines blow-by..meant to escape to the outside. But then again, that would mean a worn out engine.