Apparently Toyota already started using 0w-16 in Japan. Yeap. Zero W Sixteen. I could imagine how thin the oil is when its at 100C.
No doubt it will definitely help fuel efficiency just that it gives me the cringe about how it will perform in our hot climate.

here is an article about fuel economy vs thin oil, written by Specialists:
https://www.machinerylubrication.com/Read/518/motor-oils

But I believe engine manufacturers aren't just simply using 0W-16 without compromising engine reliability.

Even if you track your car in stock form, provided that you maintained the car reasonably, unlikely it will not overheat the engine oil. The faster you go, the cooling circuitry also works harder correspondingly. The added velocity of air also draws more heat away from the radiator and other heat sources.

It is very hard for people to accept that, unless track ALL the time or the car was purpose built for tracking/racing, if thinner oil that is within the stated spec in the manual, there would not have any noticeable ill effects (even if you track once in a while in stock form).

If you really want first hand experience, please spend some time looking through the pinned dedicated engine oil thread, very informative with used oil analysis to back up.

Also, please read have some readings valvetrain/drivetrain components and oil related articles in the "tech centre" segment of enginebuildermag website. It is quite deep for starters, but you get solid knowledge from real engine builders :
https://www.enginebuildermag.com/tech-center/