I own a 2006 SLK 280 that leaks engine oil if its been sitting too long (over a week) without being driven. I noticed this problem primarily in the cold winter months when I don't operate the vehicle very often. Could this problem be related to the contraction of these plugs?
Doubt it's weather related. Take a flashlight and rub your finger around the bottom of the plugs ... if there's a leak there should be some residual oil on your finger.
That's how I noticed the leak on my 2005 350.