Why does an I/O engine needs to operate at 140-160F

I know that hot salt water is extremely corrosive and would easily corrode engine parts like risers. And also the engine compartment must not be hot beyond 200F
But if i run it only in fresh water, and have good compartment aeration ,wouldn´t operating in 180F be better, like less carbonization on valves, improvement on engine life, less oil changings, less fuel consumption...
What is the problem with it??? Why every marine manual says : " do not use automotive thermostats, no matter how tempting it is". i cannot get rid of this phrase.

 

Why? Probably because of the possibility of the boat being run in salt water. I have 160 thermostats (automotive) in my Crusaders (fresh water closed cooling) and have had no problems, but I'd feel much better with stainless marine units in an open cooling system. We remove the thermostats from outboards down (up?) here, they clog with sea grass. Generally not a problem since our water temp is usually in the mid 80's most of the year. I'd imagine Brazil is similar? Those fussy fuel injected engines probably wouldn't like that though.

 

I recently read, it was to prevent "continuously shocking" the engine due to a large temp. differential. (cold water entering a hot engine) Although with a closed cooling system, I can't see where this would be a problem, since it's just like a car, only with a different heat exchanger.

Also, not sure what is done now, but on the older mercruiser systems, the sea water was routed thru the exhaust manifolds to pre-heat prior to entering the Engine, which should have allowed a higher temp.thermostat.

 

The motor will carbon up,due to temp.