There's no way that a properly torqued spark plug will foul due to a faulty valve cover gasket. Most plugs are torqued to about 15 ft. lbs. I suppose that it's possible that a leaking valve cover gasket could get oil on the resistor tips that mount on the spark plug, thereby causing the spark plugs to misfire, but this is a different situation. Oil cannot get in from the outside. If you plugs are oil fouled, you have worn piston rings, valve guides; valve stem seals.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.