Instead of an engine block heater

Unless you're living in Alaska, cold oil is not really the problem here.. While warming the oil *will* help it flow better at vehicle startup, it's not a huge problem due to the Prius using its electric motors to spin the engine and get oil pressure, etc, before actually starting.. The point of a block heater on a Prius is really to get the engine block itself and the coolant inside warmed up so that 1) you have cabin heat earlier and 2) the car can go into hybrid mode faster vs. burning gas just to warm the engine up.. If you do want your oil heated as well, I would rather go for one of those glue-on oil-pan heater pads vs. something that you stick inside your engine..