20w50, much too thick and will cause your engine to work needlessly harder through all temperature bands. Gas mileage drops as well, but if you don't mind going to the gas station more often, use 20w50 oil.
If your engine is worn, loose, and you are burning oil, then you can use 20w50 to help with compression and worn engine parts.
Using 20w50 in a hot climate, is based on old, dated thinking when oils broke down quicker in hot climates. Engines werenot designed to run as hot either.
Today's modern oils, can and will easily handle your Phoenix and Arizona heat. Run a 5w30, 10w30 oil, and your engine will run at peak performance, peak gas mileage, and engine parts will be protected. 20w50 NOT NEEDED, but again, if you have a beater engine, this is your oil of choice.
And, if you have one of Ford's modular engines, you don't want anything near that heavy. You want the cams to get oil immediately after the engine starts (one of several reasons 5w-20 is recommended by Ford.)
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