This question is more about my daily driver (RAV4) but also applies to my Opel.
I just moved to Arizona and it turns out it's super hot here. My RAV4 manual says that 5W-30 is preferred for under 100 degrees, and that 10W-30 is recommended for 0 degrees through over 100 degrees. Since it's now consistently over 100 degrees, I am deciding if I need to switch to 10W-30. I understand why the weight matters at cold temperatures, but not at hot temperatures. Since the thermostat keeps the engine temp at 170 degrees, the engine doesn't get any hotter here than it did in Seattle, just gets there faster. I would assume that if 5W-30 is fine for 170, there is no benefit to 10W-30. Can anyone explain what I am missing? Or is it just a matter of time until I find my cooling system can't keep up with the engine?