Would you get oil on the plugs if only the seals need replacing (as distinct from the guides and/or the valves themselves)? What is the normal mileage cycle for seal replacement and is age is a consideration as well as mileage for the seals drying our and getting hard?

What are the symptoms of needing the guides in addition to the seals, and secondly, needing the valves in addition to the guides and seals? Just continued use of oil if the prior and lesser cure didn't stop it? Must the head be pulled off to determine whether the valves need replacing?

A bit of detail from the knowledgeable board members may help in systematically clarifying this matter and would be appreciated.