What really is the difference between the type that goes into the side of the block, versus the type that you attach (magnets) to the oil pan? Which is better, and which have you used to effect?
It seems like the type that goes in the plug is heating the block itself, while the oil pan type might get a bit of the block warm, but it is really just heating the oil.
Getting the oil warm seems a good idea for a cold start, but I'm also really interested in just getting my water temps up so I have normal running temps while driving to work and back in cold weather.
Yesterday, it was 35 in the garage, and the truck hadn't run for over 24 hours. It has no glow plugs, but I know it will start after sitting out in temps as low as 28 degrees. (This is, obviously, not ideal, and puts a big strain on the starter and battery. Glow plug circuit is something I'm working on getting done.) So I took a chance and put a propane heater under the front of the block where I didn't have to worry about it catching fire! and timed it for 30 minutes. The inside of the engine bay got above 70 degrees, and the engine started almost instantly. This is a stupid way to do it, unless it really does get cold, and I have the time to sit there and watch it for half an hour.