These engines are notorious for sucking oil through the intake gasket. I have personally fought this issue on an engine. They can consume a substantial amount of oil that way and since that oil contains no octane, it can make the engine much more prone to spark knock. BEFORE you tear your engine down, check those intake ports for oil and the back sides of the intake valves for deposits. Just in case. All it takes is one or two cylinders sucking oil to cause those issues.