Best engine temp for horse power??

On my semi's we run Evan coolant.
Boiling point is somewhere over 400 degree's.
We use thermostats that open at 210 degrees.
Are oil runs about 195-200 not pulling a grade.
We do this to get better fuel millage.
Running sysenthetic oil
No proof here,but if we are getting better fuel millage I would think the oil is working better.
The only reason we are running hotter water temp is to get the oil temp up.