[Diesel_Dave]

I think an important thing to remember is that preheating actually helps the efficiency in 2 different ways.

First, is the fact that the warm engine is more efficient because there's less heat transfer.

Second, a warm engine will have much lower friction, especially upon startup.

A block heater addresses primarily the 1st issue, an oil pan heater primarily the 2nd (obviously there's some crossover too). Immediately upon startup, there is relatively little oil between all the friction surfaces, therefore high friction. The colder the oil is, the thicker the oil is and the longer it's going to take for all the oil to get where it needs to be. Also, cold oil takes more energy to shear so friction will be higher due to that as well.

This is why engines that are used for long trips last longer than ones used for only short trips--frequent cold starts increase the overall wear greatly. Something to consider is that pre-heating (particularly the oil) can help reduce engine wear in addition to helping your FE.