The car is driven about 1k miles per year, for 15 years now.
Oil changes are done once a year, and I have no seepage on the valve cover (but I do have that dreaded leak in the front of the valve box gasket - new gasket on the way).
Filters are ok.

According to the service manual, my engine (M110.986) does not have a PCV valve per se, only a passive passage for the oil vapors.

I'm thinking this could be related to a rich mixture problem that I'm chasing for the past 8 months now. According to my research, unburnt fuel can get to the sump. Perhaps this is wreaking havoc on the PCV system, leading to my problem?