A heater that goes into the freeze plug would be the way to go.
However I think it is totally un-necessary around here and won't help your mileage. You may use slightly less fuel because it won't take as long to warm up, but still not needed and the cost of it will offset any fuel you might have saved.
Having said that, it isn't going to hurt anything keeping it warm.
My boss' helicopter is stored in his barn (no heat/air) in Springfield. It gets chilly in there in the winter. Since the helicopter uses heavy oil (5w50 or straight 50w) it is very difficult to start when it has been sitting at 15 degrees all night. I had a heating "pad" glued to the bottom of the pan that is thermostatically controlled and keep it plugged in all winter. It's enough to keep the oil warm and even the engine bay. It should also help moisture from building up in there. Plus it fires right up.
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT