you're gonna want to fill in Toyota's engineers on that then (ambient temp/viscosity chart on page 337). this is for the FJ sold down under between Feb 2011-Aug 2016.

every car i've owned shows a similar chart in the owners manual. sure, oil temps settle close to water temps just putzing around town but they'll rise independently if you run the engine hard, regardless of water temp. the hotter the ambient temps, the harder it is the for the oil circuit to shed the heat and come back down to normal temps unless you take the load/RPMs off the engine or add a cooler. a good example would be towing a heavy trailer up a long grade in desert heat...extended high RPMs and constant high load.

i could be wrong but this sounds like you're talking flashpoint, not viscosity. the flashpoint of most synths is between 400-430F. that's where the oil gets so hot that the vapor around it will literally catch on fire if exposed to an ignition source. you're absolutely right though, by that temp you've turned your engine into slag anyway.

oil at 220 is ideal. at 250 degrees the viscosity of an oil will start to dive. by 280F a 20wt is at about half its designated viscosity (using this approximate calculator and this report on TGMO's 40/100C values...start the chart at 100c). bad news for oil pressure.

an extreme example for sure but shows how viscosity dips at higher oil temps in hotter climates.