BTW, we in the shop I work in refer to this as "motor snot".
It condenses there because the plastic cap and also (importantly) the filler neck are plastic and are cooler than the surrounding metal parts, such as the valve cover the filler neck attaches to.
I've never noticed this on the earlier engies, or any other current MB engines such as the 111 engine, where the plastic cap attaches directly to the valve cover.
I am unsure exactly how the moisture enters the engine to begin with, I would assume either slight amounts of normal blow-by past the rings.

Gilly

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