If you believe it is carbon buildup, try this...If you have it running, I would get it warmed up and get a spray bottle filled with water. Run it at a high idle and squirt water directly in the carb. As much as you can with out killing the engine. That will knock the carbon buildup off and out your exhaust. Just do it outside.
Adding oil will add to the compression reading of any engine. I'm not sure it indicates anything to me accept that oil is in the cylinders. Do a leak down test on the engine and determine if it is rings or valves.
If you can find a scope that will fit into the spark plug holes, you could look for oil wash. That would be where the oil washes the piston clean around the edges near the rings. That would indicate bad rings.
Also look for oil clouds coming out of the exhaust when driving and pushing in the clutch when shifting. That would also indicate a worn engine.
