Recently, I purchased a 2011 Cts Coupe (3.6l rwd). I installed new spark plugs and ignition coils. During the installation I noticed that the spark plug on the driver side rear cylinder had oil covered on the end of the plug. I am not sure how long the previous owner had been driving the car in this condition, could this issue have a long causing effect with the reliability of the engine after it's been repaired? What may have caused this problem? Any advice or help will be greatly appreciated.