A lot of people say to go up a grade in hot climates (40 grade instead of 30, for example) to better handle the heat. The theory is that higher ambient temps means higher operating oil temps, but in many years of careful observation in several different vehicles, I have not once been able to prove that theory. I've always seen operating temps remain pretty constant all year around with the only difference being how long it takes to get to that temperature. In fact, the only time I noticed a higher operating temp was when I went to a higher viscosity. This is logical sense higher viscosity means more hydrodynamic friction in the bearings, increasing the temperature delta through the bearings.