[aerohead]

An SAE paper on fuel economy,published long ago,showed the relationship between temp and mpg.
Basically,they said that you wouldn't achieve your highest mpg until driving continuously for 22-miles at 50 mph,after which viscous shearing forces would bring the fluid up to its equilibrium temp and viscosity for whatever outside temp you were at at the time.
At Bonneville,many teams pre-warm all the lubricants before a run.
Full synthetic fluids can help,and are standard for many contemporary cars.