at higher/highest operating temperature thicker oils flow and provide better engine cooling/protection.
as far as i have checked, recommended oil for latest Ford GT ecoboost turbo V6 is of 5w-50 viscosity. the reason for that is high revving sports machine.
similarly racing oils are sometimes 10W-60 that too coz, if you run the car continuesly at higher revs higher viscosity helps.
hope this clarifies since we pakistanis wanna push our 1.3 Litre GLI thinking it as a fastest car in the world. so IMC and Toyota must have thought about our mentality and driving, weather, environment and fuel conditions and decided for 5w-50 (which is not wrong considering all factors).