I bought an older 300D 2.5 Turbo recently and it had been serviced by MB with Quakerstate oil only for the past years here in Canada. Switched to synthetic as all my other cars have been running synthetic ever since I've had them too.. never looking back at that QS oil. I don't know why that's what they use at the dealerships here...

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1991 MB 300D 2.5 Turbo (207000km) (now gone)
1993 Audi 100 CS Quattro (300000km)
1999 VW Jetta TDI (290000km)
2002 Audi A6 3.0 Quattro (150000km)
2005 MB E320 CDI (34000km)