1999 Plymouth Breeze Related to oil in the spark plugs

I Just recently aquired a 99 plymouth breeze and I have been having problems with it. While changing the spark plugs, I too noticed large amounts of oil. I automatically suggested worn rings. After closely inspecting the plugs I noticed the oil was mostly on the upper outside of the plug. The area inside the plugs were free from oil burn evidence. Very clean. While reinstalling the new plugs I noticed openings right at the rim of the valve covers where the plug boots seal.I believe the oil seeps in around the p;ugs unnoticed until removal of the plugs.
I am needing a suggestion why I have a trouble code for 4 cylinder misfire. Code p0304. I replaced plugs, checked wires for fire, checked injectors operation, everything seems to operate correctly. My Car Idles great, starts great, rides great all except quick takeoff. Check engine light stays on but flashes when trying to accelerate quickly. Coil has spark on all cylinders. How does the computer read a misfire. WHat are the other possiblities capable of causing this reading.