This is been endlessly debated since Ford change the standard to 5W-20 almost 20 years ago. A few years ago I spent a lot of time on BITOG another oil forums trying to learn more, as well as perform a half-dozen oil analyses to see for myself.
In the end, the idea that thicker oil is helpful is a commonly-believed myth. The temperature inside your engine is controlled by the thermostat to a relatively narrow range after warm-up, and your engine oil doesn't care how hot it is outside. You can find more 300,000-mile examples of modular V-8's than you could shake a stick at that have seen nothing but 5W 20 their entire lives site; some of them predominantly used for towing in hot climates. If lubrication with 5W-20 was a problem you would see common failures, but you generally don't. These engines are among the longest lasting things on the road, the overwhelming majority outlast the chassis they're installed in.
I've heard the argument that thicker oil can cause damage, but it seems rare to find examples of that. I prefers 0W-20 because of the increased protection on cold starts while retaining viscosity at high temperature. I even towed 14,000 pounds across the country in 2012 with 0W-20 in the V10-powered Excursion and never had a problem.
