Air flow is not linear with speed.Double the speed,and you cube the air flow,so higher speed gives a much higher air flow than low speed,and the engine will run cooler,even though the engine is producing more heat,overall,the engine and oil are cooler.
The old air cooled beetles could run from coast to coast in 2nd gear,pedal to the metal and not overheat,but don't lug the engine at low speed,same with the old Porches.
Most diesels are also designed to run wide open throttle also.
Lugging a diesel is a death sentence for longevity.Truckers know this.
I am an old timer,and I have had a 16HP Briggs single cylinder MTD garden tractor since 1979.It was sold by JC Penny as a Model 1853,but made by MTD before they had their own brand.
Hydrostatic drive,hydraulic lift(Converted to 3 point cat 0 hitch by me.Replaced the front axle spindles with boat trailer axles and Bearing Buddies,no other problems since,and I hardly ever ran it at full throttle.Must have been a different fan design back then.
Some progress isn't progressive.