My 2002 F150 4.6 L V8 has oil on the spark plugs on number 2 and 4. I had a misfire on number 1 back in October and took it into the shop, $250 bucks later they had a new plug and coil in. Then two weeks ago i had another misfire on number four, i bought a chilton and took care of it except the spark plug threads had oil on them. I just replaced the plug on #2 IOT see if the plug also had oil on the threads and it did. Any ideas? Valve cover gasket? someone told me head gasket? Thanks