If the oil was cooled to raise the operating viscosity in order to give more film thickness, then it would essentially be like running thicker oil at "normal" oil temperature (200 to 220F), and therefore no real gain in fuel mileage would be realized. So I don't see why they would run the thinner oil at lower temperature to protect better when they could simply specify a grade thicker oil. Besides, it's bad for oil to run it too cool all the time.
Engines (if designed well) that use thinner oil should have an effective oil cooling system to ensure the thinner oils don't get too hot and thin to the point where MOFT can essentially go to zero. Especially when used in conditions that can get the oil temperatures above that normal 200 - 220F operating range.