Oil on sparkplug threads?
 

This is for a '92 190E 2.6 6 cyl. Just a little backround on the car. Several months ago I had the headgasket replaced on this car. I have been doing alot of highway driving of late. Average cruise speed about, 70. The engine seems to be burning oil. I don't see any blue smoke out the back, the engine is dry, yet I am having to add oil with increased frequency. I just checked each sparkplug and they all looked good, electrode wise. Consistent color, no oil sludge/build up, or soot. However 4 of the sparkplugs had oil on the threads. I did an oil change earlier today and will be monitoring/tracking the oil usage, but for now I am looking for ideas as to why there is oil on the threads.
I do have guess on the oil usage and being on the threads, and want others' opinions. I am guessing that those respective cylinders have leaking valve guides, the oil leaks into the cyl., but the sparkplug burns hot enough to burn the leaking oil, cleanly. However under the compression stroke the oil is being forced up into the threads. With the sparkplugs out, there was no evidence of oil in/on the seat where the sparkplug sits in the head, just on the threads.

If it matters, my highway fuel economy has been a consistent 26-27 mpg. using premium gas.

Mine has some oil on the threads when I pull them too but gets 20+ mpg and runs perfect. I'm not complaining. :D

Oil on the spark plug threads is likely from a leaking valve cover. It didn't get on the treads from inside the cylinder. But bad valve guides and seals are the probable cause of your oil consumption.

I would have expected that the valve seals and guides would have been done when replacing the head gasket - No?