Ulli, Being it that both of your cars have gasoline engines, I don't know why your oil would be "black" that quick. If it were diesel that would be normal due to the carbon. How is you sel running? Maybe your engine is running too rich, how is your fuel economy?

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member