Hello all, I was asked to change the spark plugs on my friends 2001 GS300. While changing the plugs I noticed oil on the plugs (not the top, on the threads). I have read people talking about changing the valve cover gaskets and the spark plug tube seals. How would valve cover gaskets allow oil into the spark plugs? I have also read people talking about replacing rings and or the head gasket. Wouldn't there be other symptoms (smoke, overheating) if it were one of those two problems? This vehicle is only having a sporadic misfire. This is the first Lexus I have worked on, so any helpful information would be appreciated.