Cylinder #8 misfire

[eirechef]

On my o2 ex..I have 185k on it. The service engine light came on and it was running rough .the code was #8 cylinder misfire. I changed the coil pack. There was an oily spark plug. I cleaned it off and drove the car and the ervice engine light went off and it rode fine....for a while. a few weeks later the same issue occured. Light came on, running rough and the code again was #8 cylinder misfire. I tested the coil pack on another cylinder and it was fine. I replaced the plug, it was still oily. It cleared things up for about two weeks and now I have the same issue again with #8 cylinder misfire. What can I do next? Is the oily plug the issue? where is the oil comming from?
Thanks in advance for any help on this
Simon

 

[matt0248]

Where is the oil? on the upper porcelan part of the plug? That would indicate a vavel cover gasted leaking oil into the plug hole and filling it up until is gets in between the coil boot and plug, esentially shorting out the plug. IF the oil is on the firing end of the spark plug it could be caused by worn valve guide seals, worn piston rings ect..