Not going to change a thing. If the thermostat opens @180 or 195 it's still going to be the same end result. If your engine temp is high, regardless of the conditions, it's time to diagnose the coolant system.

 

Yeah...…...NO. Sounds like a misapplication of theory. And the earth isn't flat.....

 

Thanks for that Nostrodamus...

 

The point is that the thermostat controls the running temp of the coolant...unless the cooling system isn't capable of maintaining that temperature. If it's not, the thermostat setting is irrelevant.

To your point, there is actually a situation where a lower temp stat would be helpful in stop and go traffic. If airflow increased enough during "go" periods, dropping the coolant to a lower temp than the factory 195ish point, it might buy you a little more time before overheating occurs when you stop again. Not a huge benefit, but a slight one.

 

No problem so far, drove with the big heat we had ( 40°) to France and did 1600km. My rig was fine, gas mileage too.

 

Did you check the condition of the engine oil? Post a pic of wipe off dipstick on a white paper towel when you get a chance.

 

You are not going to convince him of anything, ever. Even with all of the facts.
Sorry to say. This one is a lost cause.