We can answer this! A leaking valve cover gasket is the likely reason why there is oil on your spark plugs.
If any of your seals and valves are leaking, oil can slip past and end up on the spark plugs. From the O-ring seal to the head gasket, there are many possible sources of the leaking oil. Whatever the cause, it’s not good—and you can expect to pay between $150 to $1,600 to repair the damage.
There is a very slim chance that a damaged piston could be at fault, but this is rare.