[JonA]

If you have oil leaking onto the outside of the exh manifoild it is the cam tower gasket. Put that on the list and do the little rear gasket and the camshaft seal at the same time.
From the pictures it almost looks like the oil was on the part of the plug outside of the engine, like it was pooling on top of the plug. If the plugs are not tightened you can get over time a little blowby that will coat the outside of the plugs with a oily substance. Or, and this is a long shot, the other side of the cam tower gasket might be leaking and the oil is finding it's way down into the plugs. I would guess if that were happening you would see the oil on the outside of the head when you pulled the plugs.
Do a compression test, do a leakdown test. Pressurize the cooling system and see if you lose any fluid.
Dont pull the head for a new gasket gasket until you are a little more sure that is your problem. That's a can-o-worms you do'nt want to open unless you have to. You don't have a lot of the big indicators; milkshake, overheating, big loss of coolant, hydrolock.
Keep us posted. Good luck.

Jon