One of my past careers was a CAT mechanic in Antarctica....I have some experience with coolant and oil heaters.
The 'cleanest' install would be the immersion heaters, or freeze plug heaters. If you can plug them in while the coolant is still warm they maintain the temps very well. If you plug them in to warm a very cold engine they take quite a while, depends on how big the engine is and the heater wattage. But they do work well, especially if you can put one on each side of the block. And the ones we used, either Katz or Zero Start, had removable cords.
I would NOT use any open-element type of oil heater, they burn and crystallize the oil, it turns to a big carboned-up mess and can't be good for the oil. It seems to heat only the oil touching the element and bake it to the element. Doubtful that a dipstick heater could A) have enough wattage to really work and B) safely heat the oil.
Most quality oil heaters are enclosed in their own tube so the oil doesn't directly contact the heating element. Again, if you plug in while the oil is warm it is easier to keep it warm, and the warmth will migrate up the crankcase to a certain extent. These also have a thermostat option and usually require welding a bung in the pan; so you thread the heater in one bung and thread the thermostat right beside it in another bung. Probably way too complicated for just a passenger car.
The external tank-type coolant heaters work well if you can get them plumbed very low on the engine for the draw and return the hot water up high so the movement of the water goes through the entire engine.
They make some really high wattage tank heaters for diesels and they work very well. It is best to use the thermostatically controlled ones, but that increases the size of the install. They work well if you have the room and the water ports for an install. Heating the engine coolant will also help warm the intake so the fuel atomizes and burns right away for less wash-down.
I've never used the magnetic pan heaters, but if you could use one in an area where the wind won't negate the heat from it, it might work to keep the oil a bit warmer.
Putting a magnetic heater on a pan out in the open would be less effective than one on a car in the garage.
The one other heater is a battery heater. There are blanket-types that wrap around the battery and pads that the battery sits on. It is my opinion that the pad-types work a bit better than the blankets because of the direct contact with the battery, and the heat rises up through the battery. The blanket theoretically will keep the battery in a warmer environment, but they also seemed to burn out quicker (just my experience).
Having a warmer battery makes for a faster spinning engine, which is a big help on a cold diesel.
Just some of my experiences.....we would have engine coolant heaters, battery heaters, and oil heaters for the engine, trans, and hydraulic tanks of the dozers and loaders
Bill
