From what I've read on the web, the base stock comment is correct. However, if you already have oil leaks and old/worn gaskets, synthetic oil will make it worse. If you have no oil leaks, you should be fine. I'm using Mobil1 15W-50 in my 300SDL. No leaks or lose of oil. My car now has 220k. Better start ups, increase in fuel economy are some of the things I've seen so far. Dipstick is clean also. Not as dark as the dino oils used in diesels. preference is the key. There are numerous colors in the rainbow. Take your pick and stick to it.
I change my oil every 6k and the filter every 3k.

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99 C230K Sport
87 300SDL
81 300SD