I replaced the Spark plugs on Babyblue yesterday and she's running real good, purrs like a kitten.Using a flashlight I looked into the spark plug holes and did notice there was some carbon buildup on only one of my pistons, much more than the others. The spark plug I removed from that cylinder also seem to have what looked to be oily residue on the plug screwing threads. What's going on here? should I be concern? The car has 184k miles.