NAPA has Kat's dipstick heaters that are rated at 90 watts. Way less than a block heater inserted into a core plug would be. In real cold weather it would need to be plugged in overnight to do much. My diesel has a 1000W heater in the coolant system, which works in about 3 hours to get up to a reasonable temperature. I agree that sticking 500 watts down a dipstick tube would be a trick and probably not that safe either.
