I run 2x 600 watt immersion heaters, one on each side of the block. I use only one until it gets close to 0*F. I use both when it's like -40*. I leave mine on overnight if the roads are real bad, because there's a good chance I'll get a call in the middle of the night to go get somebody who ditched themselves. Otherwise I run both starting at 4 AM on a timer, Start the truck at 6:50 and drive away around 7:00 - 7:10. I run the heaters and the truck longer as it gets colder. The advantage to having 2 heaters is that you have 2 heat settings, you have a little redundancy (if one fails you'll still have 600 watts of heat and will probably be able to get your truck started that day) and if your glowplugs fry - you can move your truck indoors and plug it in for 2 hours (keeping an eye on it) and the engine block will warm up enough to start without glowplugs (ask me how I know
).
Imersion heaters are 10X better than anything that connects to a hose IMHO. It was frequently -30*F and colder last year where I run our trucks. An inline heater might work fine somewhere it's not so cold, but too much heat is lost in the hose when it's -40*. Immersions are safer too, since the heat elements are under coolant where they can't start a fire (although the power wires should still be carefully routed & checked often to avoid trouble).
Pan heaters are for heating Oil, not Engine Blocks. They may heat the block a little by heat conduction if you live somewhere warmer, but here - not at all. In case anyone didn't know, warmer oil (when it's real cold out) helps prevent wear to your engine when it's starting by keeping it thinner and having it flow faster to where it's needed. I don't have one yet, but I am looking for one. I would say for "getting it started when it's cold" you want a block heater or both, not just an oil heater.
I would 2nd the "no stick on heaters on the fuel tank" rule.. Around here, the fuel companies add the winter additives for us (and charge $0.10/Quart more) so our fuel doesn't gel up. I'd recommend the chemical fix rather than heating the tank with electric heaters. Sounds safer and not much more expensive. Good luck!
