Here's the deal: 'Multi-weight' or 'Multi-grade' oils are formulated to have specific viscosity, or flow characteristics at certain temperatures: a 'cold' temp', and a 'hot' temp'. The higher temp' is specified in the test standards, based on engine operating temp's but, fast-forward ~50 years, and that's become even more variable.
Todays engines are designed with tolerances, and part of that is to allow for oil flow through, and between moving parts.
When the oil gets hotter and thinner, it may not maintain a film layer between moving parts, however, if you use higher viscosity oil, you may have a problem with not having adequate flow when it is at normal operating temps...
If you are running in the heat all the time, you are still probably better off addressing that: installing a different thermostat valve, as well a discreet temperature gauge, upgrade cooling fan under the hood, and look into getting a lower temp activation switch...also, look into changing/adding radiator capacity.
Keep the engine temp correct, and stick with specified oil.
Good luck!