The car was misfiring while accelerating from a stop. Then it started doing it while idling. Then it started doing it while in park or neutral. And it started misfiring when you turned on the A/C. It jsut got so bad I had to do something about it. There was about a half of a teaspoon of oil inside the #4 spark plug tube. I pulled the spark plug out and the electrode and threads were also wet with oil. I had replaced the valve cover gaskett and spark plug tube seals at around 120K miles. And with the car approaching 200K it started misfiring as I described. The spark plug tube seals do not last over time… They eventually leak oil into the spark plug tubes. So what I did was I bought some high quality RTV gasket sealant and ran a bead of it around the tube seal surface that mates with the valve cover and I also ran a bead of RTV on the bottom side of the valve cover as well. I waited about 5-10 minutes for the RTV to become tacky and I placed the tube seals on the tubes and put the valve cover on. The valve cover squished the RTV into the tubes and I used my finger to smooth it out so I could get the spark plugs in and out. Now it is sealed against leaking. I should have done this when I did the Felpro kit!!! Well the problem was immediately solved! 
No More Misfire!!! I will also add that the oil clearly Eroded the spark plug electrodes which were NGK Platinum. The electrodes on this higher end plugs is susceptible to erosion and doesn’t last… Then you will get misfire just from the plugs being unevenly gapped. So get NGK plugs with a thick Electrode. NGK R type only. So I hope this helps every one who gets a P0300 or misfire code.
Post time: Jun-22-2017