Improper timing, mixture is rich, engine wear.
If you are talking about oil on the outside of the spark plug then you have a leaking valve cover gasket. Oil on the spark plug electrode is a sign of defective rings and an engine in need of an overhaul.
if the oil is at the spark plug base the valve cover gasket is leaking. if the spark plug electrode is covered with oil the engine needs rings
to much oil or spark plug is bad
No there should not be any oil where you plug your spark plugs into
valve cover gaskets
replace valve cover gasket. there are 4 small rings that seal the plug ports. this keeps oil out of the spark plug port.
might have Ring damage.
You have an oil leak
Oil deposits on the spark plug is caused by poor oil control. Oil is leaking past worn valve guides or piston rings into the cumbustion camber.
If you mean on the exterior of the spark plug then the valve cover gasket is leaking. If it is on the electrode of the plug you are burning oil.
Poor oil control-worn valve guides letting the oil into the combustion chamber.
Screw out spark plug then screw in non fouler then screw in spark plug in non fouler.