Oil on spark plugs

[Jim Wright]

I finally got around to changing my plugs today as they were long
overdue, and on two of the cylinders there was oil on the upper part
of the plug and the threads, but the electrode itself seemed fine.
If I'm remembering correctly, there's a rubber seal around the
openings for the plugs in the valve cover, I'm assuming that the most
likely fix for this is to replace my valve cover gasket and the spark
seals?  Anything else I should check out while I'm in there that may
be obvious?