As Longston said, use what your manual calls for. My VW owner's manual says to use 5W-40, but 5W-30 can be used if 5W-40 is not available. So "40" is the perferred weight. My '80 SD's manual says to use 15W-40 for temps above 23 F. Below that, use 10W-40 or 50. Again at least "40". Domestic and Japanese cars call for anything-30. It seems everything from Europe calls for the heavy oil. Look what Porsche is factory filling their cars with, 0W-40 Mobil 1. There's that number again, 40.

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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