The spark plugs wont be a reliable diagnostic tool in my case. Because of the rich mixture problem, I clean them very often (carbon build up form the excess fuel). I haven't noticed oil build up on them.

And yes, the oil is wet. Is a puddle about 2mm deep.

However, I have found that the vacuum hose form the intake manifold to the WUR was disconnected (it's old and dry), and perhaps this may affect the ability of the engine to suck the oil back into the combustion chambers (because of less vacuum).

On a side note: whoever the engineer is at Mercedes that thought that putting all those air/vacuum hoses on the most inaccessible place possible was a good idea, deserves to be shot!