If oil is on the threads it wasn't in the cylinder to begin with... thus, yes, it came from outside the cylinder. As I said, the cam cover gaskets are a VERY common leak point. Age and over tightening by previous owners causing shrinkage and compression as well as the screws backing out over time are the most common sources of the problem... sometimes you'll find one where the cover is warped or even cracked due to someone getting a bit too overzealous while tightening them down. The torque spec is 22 INCH pounds, That's just a good, firm HAND TIGHT with a phillips screwdriver (make sure you use one the right size for proper transfer of power... a #3 IIRC). Some people recomment swapping to bolts as the only way to go... they're NO better at all.... just easier to get a solid grip on, but they're also easier to overtighten, especially to the point of causing damage.
Whatever fastener you use put on new grommets and if you like, some blue loc-tite and tighten to spec... no more.
EDIT: Oh, and other causes include a crack in the head or a leak at one of the three bug hex plugs in the center or a crack in the PCV plumbing.
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