I would like to assist you with your question. There are a couple of reasons a spark plug can become oily. The first reason is from the piston rings and cylinder being scored and oil leaking past them while the engine is running. Engine compression is usually adversely affected by this and the engine tends to lose power. Another reason can come from overfilling the engine with oil. This causes the crankcase ventilation system to blow oil out of the diaphragm and into the intake. A failed crankcase diaphragm can also cause this issue.
Is the engine smoking while running? There are a lot of possibilities and if you would like to reply with any other details, I would be happy to narrow it down to the cause with you.
Please let me know if I can assist you further.