I disagree on all of the above.heating the coolant is a LOT better then heating the oil. Heating the oil can burn the oil. Besides, warm oil doesn't help the engine start.
heating the coolant using a block heater means the HEAD will get warmer. Most of the water circulates up to the head.
Then when you crank the engine over to start it, the head is warm, top of the block is warm, vavles are already warm, and the truck will start more quickly.
I have been using an oil pan heater for years on my truck - never a problem with burnt oil and warm oil DOES help the engine start. The oil heater helps improve oil flow to the HEUI injectors, allowing for easier firing.
I also have used the OEM block heater, again easy starts compared to no plug in.
The main difference I noticed in the oil heater vs. OEM block heater is 500w (oil heater) vs 1000w (block heater).
From any energy consumption viewpoint, the oil heater is better.
My uncle who has been a diesel mechanic for years, has always used oil pan heaters on dumps, bulldozers, tractors, etc. He swaers by them. He always told me if you got a good functioning glow plug system, no need to worry about combustion chamber heat issues.