Use what the owner's manual calls for, based on outside temps. I know that the owner's manuals for both of my Benzes suggest 10W-40 as a year-round oil. However, I use 15W-40 since it is a diesel rated oil.

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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