How many miles are on the engine and what year is it? Carb or fuel injection? First thing I would do is a compression test on the motor. Might be blowby past the rings, letting the air/fuel mixture into the crankcase. After you do the inital test, write down the readings for each cylinder, then squirt some oil in the cylinder, and do the test again. If there is a differance in the two readings, then you need to rebuild the engine.

My .02

Rod