I have a 2006 Focus ZX5 on which I recently changed the spark plugs. 3 of the 4 cylinders had oil on the threads and in the wells. I checked the oil and it was full. It doesn't smoke, sound bad or anything of the sort. It still seemingly runs great. I did a compression test with the car completely cold and got the following results:
Cylinder 1: 240 -- 260 (wet) This is the one that didn't have oil on the threads when I changed the plugs.
Cylinder 2: 230 -- 260 (wet)
Cylinder 3: 240 -- 265 (wet)
Cylinder 4: 235 -- 265 (wet)
Based on my Googling, those results don't seem to indicate there is a problem with the pistons/rings nor does it seem to indicate a head gasket leak. Based on these results, along with there being a little bit of oily grime in the center cavity of the valve cover, I'm leaning towards it being a valve cover gasket leak.