HMMmmmmmm... do I sense another oil debate. I agree with Jim, that mechanic is blowing smoke. Castrol is good oil, it's Quakermistake, I mean Quakerstate and Pennzoil that you want to stay away from. That mechanic may be telling you to use Quakerstate because that's what his dealership sells. A lot of dealerships will recommend what ever oil they can get the best deal on, providing it meets MB standards. Now there is a difference between just meeting a standard and exceeding it. I mean, we all could live on bread and water, but what is best? I used Castrol GTX for years. I don't used it now because it doesn't have a "C" rating for my diesel.

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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