Dears, I have a Ford Mustang V4. and I live in Saudi Arabia, and the weather is very hot. Ford recommends full synthetic oil, 5w30, and some engineers here say that it is not suitable for our climate, and I do not know what is suitable, especially since many people have had problems with the turbo.
It gets pretty hot here in the USA too. The highest recorded temperature in Saudi Arabia was 52.0C in Jeddah. The highest temperature ever recorded in the world was here in the USA in Death Valley, California at 56.7C. Cars drive through Death Valley every day.
Phoenix, Arizona, USA, population 1.6 million, has temperatures over 45C most years and as high as 50C some years. Ford's oil recommendations take this into account
I checked the 2015 Mustang owner's manual, and Ford says that 5W30 full synthetic oil is recommended for your car in all climates, hot and cold, winter and summer. It is best to follow the manufacturer's recommendation.