C) Rob suggested valve seal and you seem to be lost on that. The seal/guide (if I understand him correctly) to which he refers is circular, like a donut. Through the center the cylinders valve stems slides and it is mounted in the cylinder head. It helps seals the cylinder for compression build up and prevents oil from dripping down into the cylinder. Test by using compression gauge on cylinder #6; then compare with another cylinder. You will also be testing the valves sealing properly at the cylinder head itself. If the guide is not sealing then oil is running down the stem to the cylinder causing bad seal and the dirty spark plug you mentioned in a previous post.