It's always amusing to see the confidence people have in asserting things they cannot possibly know to be true. A thicker oil than what is recommend almost certainly will not void your warranty, but it would be childish to say that it's impossible for it to happen. But it's also childish to assert that it 100% will void you warranty. You can't possibly know this, so stop asserting that you do. There's no certainty in either direction.
What we do know is that manufacturers are scraping every last fraction of a mile per gallon out of the engine to try to please the government. And of course, the government has no durability requirement for the engine nor any rules around oil consumption. So, if you are the OEM and have to choose between placating a government that holds the power of life and death over your business and placating a corner case customer whose engine is an oil burning disaster in only 80k miles, guess who wins? It's not even a contest.
SO not only are we getting thinner and thinner oils, but then we get all the bandaids required to make them work acceptably in an engine. Clearances must be tighter than ever, with surface finishes that are smoother and flatter using materials that are often harder. Since you need more oil pump to pump that thin oil, you have to go to a larger oil pump, which would offset the tiny fuel economy advantages if you don't make it a variable displacement pump.
So a modern engine that is more robust to thin oil (note carefully the phrasing here) can often use a thin oil and produce acceptable life for the majority of customers. Obviously this is true or Toyota wouldn't wager their warranty spend on it.
But being robust to thin oils is one thing. Requiring them is quite another. Now I ask how the same engine can have such wildly different viscosity grades recommend in different countries. It seems likely that the engines are all the same and that they don't tailor clearances for different regions. It seems likely too that the thicker oils allowed in the absence of CAFE coercion are closer to the true ideal for the engine in terms of wear performance.
If your emotional needs are such that you must slavishly follow the country-specific recommendations for your manual, then follow those recommendations. But it is indeed an emotional choice, not a logical one. So don't be surprised when those who are less emotional and a bit more clear-eyed point out that thicker oils--within reason- are perfectly safe and may even often protection advantages over the thin oil that is your Warranty Woobie.