Ford: 98 F150 Valve Cover Leak?, valve cover gasket, head gasket

Question
I own a 98 F150 with a 4.6l engine. It has been stubling a idle lately. I took it to the dealer initially and they told me that was normal. However, I decided to change the plugs and found the back plug on the passenger side left an oily film on the outside of the spark plug socket. In addition, there is a noise that seems to be coming from the same area, but only when the vehicle is under load. I have taken it to a couple of places to get an estimate for repair, one telling me that its the valve cover gasket and another telling me its the head gasket. Can you provide any information?

Answer
Steve the best thing to do is 1) clean off the eng. 2) go to napa and get you some oil dye and put it into the eng.3) rent you a black light and then this way you can tell exactly were it is comming from.then let me know and we will take it from there.