I recently had the cylinder head of my 103 engine rebuilt. At that time I felt the bore and decided there wasn't enough wear to do the bottom half. After I put the head back and noticed the spark plugs are little sprayed with oil still. My mechanic suggested the oil rings could have been carboned up. My question is what is the best way to clean the oil rings without tearing down the engine? Additive? High detergent diesel motor oil?