When changing the spark plugs what causes the old ones to be covered in oil?

AnswerIf you are talking about oil on the plug body then you more than likely have a valve cover gasket leaking, that needs replacing. If you are talking about oil on the plug electrode, then your engine is burning oil.

You should have your valve cover checked. The same thing happened to me two weeks ago, and they replaced my valve covers and spark plug wires and i haven't had any other problems

If your plug electrode is coated in oil, the actual cause of this is likely a failed valve stem seal. When this seal goes bad, oil can leak along the shaft of the valve, and into the cylinder, where it then can coat the spark plug, and generally make that particular cylinder run like crap. Incidentally, if one has gone bad, it's generally a good idea to change all the seals.

Also, if oil is on the electrode side you may have a blown head gasket. If the car has been driven while overheated or is regularly driven at high RPM (as in racing) then a blown head gasket is a likely culprit. In either case a compression test should be done. Each cylinder should reach about 155 psi.