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I have a 2010 w207 e550 and have been having oil only on spark plug threads for years. I’ve also had very light and intermittent misfire feelings that have been getting worse over time. I believe the two are connected and was wondering if the m273 engine has spark plug tube seals that could be worn or if I have a bigger issue at hand? The valve cover gasket itself sits much higher than the threaded part of the spark plug and the spark plug well is dry so I’m assuming I don’t have a valve cover gasket leak. Here is a picture of what most they plugs look like and they were changed recently. But it will gather this level of oil after cleaning it and driving it for week or so.
Sorry, I don't have a good answer for you (just a disclaimer, so feel free to disregard you wish).
But I wanted to add my related experience with this issue. I have a 2010 550 coupe as well. I bought it last year with 17K miles on it, and changed the plugs at around 18,500 miles (original plugs, I'm sure). The plugs I removed also seemed too dark-colored and a tad little oily for the low mileage of the motor (though not to the extent of the plugs you showed), The car now has about 22K miles, but I haven't bothered to check the spark plugs to see how the new ones are faring. My motor doesn't have a misfire (I don't think), but it doesn't seem to idle as smoothly as it should. It's pretty smooth, but not perfect. Maybe that's just how it is; not sure.
I also changed the crankcase oil separator and associated drip pan just in case, but the old ones looked fine, so I doubt that was the underlying issue on mine. It might be worth looking at on yours though, since the inside component of the oil separator is know to stretch out over time.
I previously had a 2008 CLK550 (same M273 motor), and I don't remember the plugs being oily at all at 65K miles (but that was over 5 years ago when I changed them). I don't think there should have been any oil whatsoever on spark plugs from an engine with 18,500 miles on it (but there was a bit of oil on them), so I'm not sure what to think.
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