Thicker oil for summertime??

According to the car manufacturers, yes — but don't go too thick. Most new cars today are supposed to be filled with 5W-20, which is pretty thin. Car manufacturers usually list 10W-30 as acceptable for summer temperatures. Thinner oil has less friction inside the engine, which is good for fuel economy. Engines have been designed to run on this thin stuff for a number of years, with improved surface finish and low-friction parts like roller camshafts.

Check your owner's manual for the viscosity recommendations for your vehicle. Older cars may require thicker oil, but it's still not a good idea to just pour five quarts of 40-weight in just because it's summer. Thicker oil takes longer to start flowing properly when the engine isn't warmed up, and can also reduce fuel economy. If you live in Phoenix and tow a huge trailer, the temptation is to use thicker oil to keep it from thinning out excessively and damaging your engine. Unless you've got an older car, made in the era of 20W-40 or even straight grades, don't just change your oil's viscosity up arbitrarily. Either switch to a synthetic, or install an aftermarket oil cooler, or both.