Odd, The oil on the Plug is on the bottom. The plug wire is dry, It's also dry on the inside on the plug wire. How could oil get to the "top" of the and inside the plug wire? If i am not mistaken when you pull the plug wire out, The area about the spark plug does not collect oil. Its clean and dry. The oil is on the bottom of the plug and on the threads, Oil in a puddle on the top of the piston.
What your telling me makes Zero sense. Simple because oil does not travel to the "top" of the plug wire and above the threads, Where there is the seal. That area of the motor is Hollow.
For one there is no need to become defensive, being rude to joe about him giving his 2 cents about the issue then saying "I will wait for someone who knows more about it" doesnt make people want to help, especially Joe. In fact, he even asked if the oil was on the threads and you wrote him off. What I said made zero since because you never said anything along the lines of "there is no oil on the threads in the head" you said "there is oil in the combustion chamber" figuring out problems always comes down to details.
Further more, you said there is oil in the combustion chamber, you didnt say it was on the tip of the spark plug you didnt say you looked down the hole for the spark plug and that the threads were dry. Both Joe and I gave you advice based on previous experience and in dealing with oil on the spark plug, 9 times out of 10 it actually is a valve cover gasket.
In this case, however, if you really are seeing oil coming from inside of the combustion chamber then its time to tear down the motor and figure out where the issue resides. The only other likely thing I can think of with the zetec motor is a head gasket or a borderline failing head gasket allowing for oil to get into the combustion chamber, however like you said this usually means coolant mixes with the oil, and that doesn't seem to be happening. I still agree with Joe, it doesnt make sense that the oil is wet in there, usually its burn or corroded because of burning the oil off, not just wet in the combustion chamber, which makes the issue a little more difficult to diagnose. Like I said in an earlier post, valve cover gasket failure is common the zetec motor.
And Joe, you are partially right about the "o-rings" the duratec has separate o-rings around the spark plug holes that are individually replaceable, the zetec has the o-rings built right into the valve cover gasket.