Any oil will flow more freely when heated, but that doesn't mean they lose their lubricating properties. 15W-40 is the recommended 'weight' for most M-B Diesels, but depending where you live, 20W-50 is OK in hotter weather and 5W-40 can be used in colder climates. Any choice though should be a Diesel Rated oil (minimum CF rated).

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown