I dont think a glow plug would burn out if it were emresed in flowing cool oil, however, there are better ways. I know that it sucks waiting for the motor to warm up, but exhaust is also a possible instant heat source. Glow plugs also take a lot of juice to run. There isnt enough juice in 2 batteries to be keeping oil hot for god only knows how long, and then starting the engine. The most hassle free idea is probably to use engine coolant to heat everything, but if you demand instant heating ability, i would start to mess around with a heat exchanger from the exhaust before putting electric heaters running off your 12v batteries.