Thinking about the reverence there is for M1 15w-50 as the ultimate performance oil. This speculation seems, in part, to be based on the assumption that the 15w cranking test is the lowest it can pass. I know it has exceptional pour-point for a 15w (-49f). So, are we possibly kidding ourselves to think of it as a 15w-50? Is it more like a 10w-50 or even a 5w-50? Maybe Mobil took the track of marketing a "low-spread" oil for performance and 5w-50 Castrol took the "wide-spread" approach for oils whose viscosities are not necessarily much different. Just something to think about. Theory is mostly all I can offer the board.