As far as 3% fuel savings, I'm guessing a TDI might do better than the big rig diesels- so many other factors, including mass, wind resistance, etc. in our favor. I hadn't thought of it, but Dieselfuel is right- running a zero pressure (waterless) cooling system means no heater core blow outs.
I know my fuel mileage on my 1.6 IDI spikes up on really hot summer days compared to winter mileage. In the winter, the best I've ever done is 46MPG. In summer, I get at least 55MPG when the temperature is over 85F over the exact same long distance route(I've often gotten over 60MPG) How much of that MPG increase is due to higher operating temps, and how much is from other factors,(warm air is less dense, hense less drag, tires are hotter/higher pressures=lower rolling resistance, etc) I don't know.
Regarding oil temperatures:
All oils have a specific continous duty operating range temperature stated by the manufacturer. For example, Mobil synthetics
http://www.mobil.com/australia-english/lcw/audiences/synthetic_v_mineral.asp are designed to be able to run continuously at 356F (180C). Since engine oil temperatures almost always stay within 20F of the coolant, running our coolant at 215F, 230F, or even higher, should not affect a properly rated synthetic lubricant.