Oil can foul your spark plugs and cause poor performance due to lots of reasons. While worn-out valve cover seals happen to be the easiest reason to fix, the troublemakers could also be: Incorrectly sized piston rings Wrong valve guides New or recently overhauled engine [Source: Denso Austral...
always use a good used spark plug, making sure it's not worn out. A worn out spark plug will have a worn or rounded center electrode and or side electrode. New spark plugs are hard to read accurately for mixture checks, but if you have to use a new spark plug, then make sure to...