In other words it's most likely that the spark plugs have oil on the threads because there was oil pooled up in the spark well when you took it out. This is caused by a leak in the rubber donut gasket that surrounds the spark plug well. Each valve cover gasket set, left and right, comes with the gasket for the outside perimeter as well as four rubber donuts, one for each of the spark plug wells. Under normal usage there's no reason to ever worry about rings or valve stem seals on that motor. Valve cover replacement is a relatively minor job.