Completely sympathetic to the notion thicker oil would be needed for hot ambient temperature conditions as for decades this logic has been drilled into our limbic automotive brains but that is not how modern engines are engineered today. It’s not as if every engine manufacturer would immediately run thicker oil if it wasn’t for the EPA, some may but I doubt it.
Modern engines are designed from the ground up to operate on thinner oils not only for fuel efficiency standards the EPA demands but also to reduce internal frictional losses and increase overall power output. Everything in a modern engine from base metallurgy, bearing and piston ring clearances, oil gallery dimensions, oil pump running pressures, oil cooling for turbo bearings, etc… is optimized for these thinner weights. Introducing thicker oils or the wrong specifications can over time increase wear and performance issues. It’s true modern engines compared to older types run more highly stressed overall but they are designed for it.
I fondly remember and miss the days of switching weight of oils between summer and winter to make sure your engine was properly protected but in terms of engineering it’s like comparing a Sopwith Camel to a modern fighter jet. Can you put 5W-40 in B58 engine and have it run? Most certainly you can but introducing thicker oils into an environment entirely designed around thinner oils could actually lead to improper lubrication/cooling of vital internal components and increased wear.
Interesting side note, large airliner engines oil viscosity is considerably thinner than any automotive engine oils. It’s basically like water and think of the temperatures/pressures it sees. In fact, the primary purpose of oil in a jet engine is for cooling of the main bearings and lubrication is a somewhat lower priority. Mainly because jet engines don’t see the same type of shearing or sliding friction a piston engine does but again it all comes down to what the engine was designed for as to the type of oils used.
For the warranty period sticking with factory recommendations would be best but do what you feel is right for your vehicle and operating conditions but incorrect lubrication of any variety certainly isn’t going to prolong engine life.
