Subaru oil on plugs

Hello! I was recently doing a tune up for my 2002 Forrester and when I pulled the spark plug out of the housing there was oil present on one of the 4. My “assistant” thought maybe I just needed to change a gasket for the housing (not sure what the metal housing is called) …but I was wondering if anyone could tell me if it could be a sign for something more serious. gathering info, books & parts to decide if I can do it myself or should call in the professionals.

Was the oil on the electrode, the part inside the engine, or was it on the ceramic?

If the oil was on the ceramic, you need a new o-ring that seals the spark plug tube. The o-ring will be part of a valve cover gasket set.

If you have oil on the electrode you have a larger problem.

My guess is it’s just the o-ring.

I was pulling the right ones and my assistant the left. It was the lower left that had oil and by the time I saw it it seemed like oil on both the ceramic and the electrode… we wiped down what we could reach and did not see any more. It was only on the one however (lower left me facing the car)…not sure if that helps.
any idea what the “bigger problem” could be and if I should stop driving it?