Because I’m pretty sure I’m smart enough to look at the graph provided that correlates ambient temperatures with viscosities. And I didn’t say it trumps an American owners manual. My Corolla’s not going to know if it’s in Speedway, Indiana or Brisbane, Australia. 90 F is the same anywhere. The engine is the same. So if I feel like running 20w50 in the dead of summer (I’m not going to) I know I can without harm. That’s it...your mileage may vary.