The truck is a 1999 ML320 US-spec gas engine. It was suggested to me at another post that the most likely culprit is the chain tensioner seal. At this point, I see no other option but to look further since I'm stuck and simply cannot identify the leak just by eyeballing the top and bottom of the engine bay. I was thinking of disassembling the belts, hoses, waterpump, etc. Oh and the leak cannot possibly be mistaken for coolant. In fact, I just changed the oil and I can see that the oil leaking is very fresh and clean oil.