where you located? 20W-50 is as thick as you can get. I would stick with Mobil 1 15W-50 for synthetic. For dino oil, use Mobil Delvac 15W-40 or Chevron Delo 400 15W-40. These oils meet the latest standards. Though as Gilly mentioned, the standard of the day when your car was made has since been superceded.

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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