Or, no matter how often you remove your spark plugs, there's always a coating of oil on the threads. If any of these situations have a familiar ring, you can blame it on leaking valve-guide seals.

Some heads were later outfitted with a rubber wiper seal which fit over machined valve guide bosses. Engine builders attempted to go even further, and they machined the guides to accept even more ...

Also, when I remove the oil cap ... a sign that oil is leaking through the valve guides, because the valve guide seals have become hard and are starting to deteriorate. Replacing the seals ...