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The old tried and true preheat method is a reflector lamp drop light with a 100w bulb rigged to a pole so you can easily shove it under the pan and remove it before driving. It provides a surprising amount of heat and if your car is garaged and out of the wind it may raise oil temps 20-30 degrees. This has been used by old timers in the bitter north for a long time. The downside is having a possible ignition source directly under your engine. It could even be put on a timer so it turns on a 3am and the oil temp is much warmer by 7am.