I don't see how oil could come from the intake valve back to the carburetor unless there was oil in the combustion chamber, meaning it would be on top of the piston. Also, though less importantly, for substantial amounts of oil to get into the combustion chamber it would have to either come past the piston rings, or through the top of a broken piston. In either case, compression would be low.
Something you might try, is cleaning out the oil from the carburetor and inlet pipe, then disconnecting the breather pipe from the carburetor and spinning the engine several times with the starter. See if oil comes out of the breather pipe, and if oil finishes up in the carburetor. This should tell us for sure, where the oil is coming from.
