Finally found my problem, and it ended up being so simple.

The leak was coming from my oil filter cap - the last time I changed the oil, the replacement o-ring provided was not the correct size (it was too thin). I compared it to one from a new kit that I had, and it wasn't even close.

The key to an oil leak is to clean eveything up well enough to see the source of the leak. I was fooled into believing it was coming from oil pan, or from the base of the filter housing. These would have been much harder to repair!

I put the correct o-ring in now, and am leak free again!

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Treiberg's Collection: 1981 240D, 1985 300SD, 1992 300E, 1997 E300D, 2005 C230K