I understand your train of thought but knowing the numbers…. The first number 5 or 15 is the cold viscosity… the second 40 is the hot. So a synthetic 5-40 doesn’t offer any less protection in a hot environment. Oh And the W stands for winter.I really depends upon where you live. I ran synthetic 15w40 in my truck until I reached 33,000 miles. The cold mornings (low 30s) made the starts rather rough. My last oil change I switched to Shell Rotella Syntehtic 5w40. The truck runs much better and starts alot faster with smoother idle. However, it rarely gets above the high 70s to low 80s here.
In the south, I would not run anything except 15w40.