When I lived in Orlando and Southern California, I used 20W50 all year round, and have never had any problem with doing do. If you look at the oil weight charts, you will see that this is what the industry and auto manufacturers recommend.
Smooth running is not a function of oil weight; besides, when a 20W50 oil heats up, it is supposed to be the same viscosity as a straight 50W oil. And the higher the OIL temperature goes, the less viscose it will be. The danger with higher temperatures is that the metal parts also expand, and so you might get interference problems; i.e., your pistons can "freeze" inside your cylinders.
If you are worried about doing something that the "cheaper" cars are not doing, go to synthetic, but be aware that synthetics do not mean that you don't have to change your oil as often. They just will not break down, chemically, as easily. They still get dirty from the products of combustion, so still need to be changed.
By the way, 10W30 is for the northern part of the USA, where temperatures go lower, and do not climb as high, as in Florida. So most manufacturers and industry sources will advise to stick with the 20W50.....Andras