1994 -  4 cylinder Honda Accord - 310K

I changed the spark plugs 11,000 miles ago after
I replaced the head gasket. Lately, I seem to be getting
a rough idle that turns into a very slight vibration at higher
rpms. I thought that maybe I had solved the problem by
doing the "Italian Tuneup" (driving the "snot" out of it),
but it stills exhibits the same problem. The vacuum hoses,
rotor, and distributor all checked out fine. I haven't cleaned the
throttle plate or idle air control valve yet.

I figured I would check out the spark plugs first. The plugs
looked fine, they were not fouled and still gapped correctly.
However, when I pulled the two outside plugs, the wells that
they came out of were both filled about halfway with oil. The
oil did not appear to get past the "rubber boot" of the spark
plug wire, but it appeared that something wasn't correct. I
have sometimes in the past check the oil, and found that it would
be low one day, and normal the next. I often wondered if the oil
was "migrating" above the engine somehow. Can someone shed some
light on what might be happening here.

Would it do any harm to try an oil additive like Seafoam?

Thanks,

Gary