Toyota Repair Problems? Ask a Mechanic for Answers ASAP
It's either because the spark plug well seals in the valve cover are leaking, the plug well tubes themselves are leaking where they screw into the head(s) or both. If you pull out a plug wire and and it's wet with oil, you have to look inside the well with a flashlight and see if the oil trail is coming all the way from the top of the well (valve cover seal).
If you leave that going on, it will eventually contaminate the wires so bad they'll misfire.
What year, model and engine size is it?