GOOD! Rotella or Delo either one will serve you well. I just wanted to make sure you weren't using a non diesel type oil.

Unless you drive it mainly on the highway and/or change it real often, the black is probably just soot build up. My concern was that if it's running better, black oil AND the fuel mileage is falling off then you might have something causing it to be fed too much fuel. In a common rail this is not really likely though.

Good luck with it.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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