Vancouver, BC, Canada CGK 2018 Ltd EyeSight
It has to do with the tighter tolerances of these newer engines. When it is cold (or cooler), oil is thicker (even 0W). So when the engine first starts up, you want the oil to start circulating to all parts as soon as possible. Thinner oil helps that. It is a well known fact that most of your engine's wear actually happens on start up. And synthetic oil also provides better lubrication in tight tolerance engines.The thing is, it’s not a old generic chart. Maybe that specific one is but it’s the same data as any other I’ve found and I’ve seen nothing that says they’ve their grade criteria . However, I’m more than buying in on the engines being designed for 0w-20 but manufactures can’t recommend a variety and service bays don’t want to have to store several drums of different grades. While the 0 is great for winter months, its pointless from mid Jun through mid Sep when the sun rises around 20°C.
That said, I do agree that going heavier than x-30 might not be a great idea. I just want know why.
In contrast my air cooled aircraft engine does not have tight tolerances. So it tends to use much heavier oil. In summer, 50 weight is recommended, and in winter, 40 weight. In severe conditions, 20W-50 can be used.
Older cars (many years ago) used to use heavier weights, too, 30 or 40 in the summer and 20 in the winter and 10 for severe cold. Eventually they went to 10W-30.
Now a days, some new cars are going to 0W-16 and as low as -8. You certainly wouldn't want to put thicker oil in those engines.