Why would a spark plug be covered in oil?

valve cover gasket leaked oil into the tubes.when you removed the plug, the oil went into the cylinder. thats why it smoked and went away. Roy

There are spark plug seals made of rubber inside the valve cover,they dry out and leak a small amount of oil onto the plug,then down on the engine and burns off,the car running better than before was probably unrelated to the seals,the plugs would make the engine run better !Hope this helps ,i am a mechanic of 35 years!