0W-40 is definetly too thin for higher mileage cars, I don't plan on using it in my Benz. I may at some point, use it in my VW. Around here in NJ the Synpower I use is not readily available. I have to go to NAPA and order it. It doesn't cost anymore or less than Mobil 1. It cost me $4.69 a quart. At this point, NAPA and other auto stores don't have this weight on their shelves. I have read on another message board, VW Vortex, that Synpower 5W-40 is on the shelf out west. It usually takes 2 weeks for the oil to come in, so I once ordered 2 cases (12 quarts), just to stock up. Once I use that oil up I may try Mobil 1 0W-40, IF the local stores have it on their shelves.
Ymsin, I don't recall Castrol having a "0" oil.
There were many comments in past "oil" threads about wishing for Mobil 1 to bring over their 0W-40, so when I happened to find this info. on their website, I thought I would let people know.

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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