The oil temp is in response to the engine. Higher engine (combustion chamber) temps, to a point, lead to more complete combustion, more power, and better fuel economy. That's why 200 F. thermostats are usually used instead of 180 F. At varying loads, from sustained full throttle to startup, oil temps will, and should, vary greatly.
As to what's the optimum oil temp at crusing speed? It depends upon the engine.
I feel that the lowest temperature that will allow water to evaporate and allow the oil to reach a good operational viscosity would be the optimal. 160F is optimal for transmission fluid, but I feel that would be a bit low for engine oil, as it has to deal with a constant stream of water vapor from combustion byproduct.
[ November 03, 2003, 05:52 PM: Message edited by: VaderSS ]
How about a temperature that puts the engine oil at a cSt viscosity between 10.5 and 13.5 at its maximum operating temperature.
That way there would be an optium viscosity that balances engine wear with engine efficiency.