Notwithstanding posts related to use of grid heaters to raise combustion temperature and enhance starting, if memory serves, Cummins recommends the use of engine pre-heat (block heater) to minimize startup mechanical wear when temperatures are 25 deg F or lower. My ISL 400 does not have an OEM block heater, and I have not retrofitted the engine with same since the coach is not operated in winter temps. Based on 20 years experience with my Cummins 5.9 (Ram 2500), if the night time temps are forecast less than 40 deg, I plug the engine in. The pre-heat makes a world of difference - the engine cranks and starts as if summertime temps prevail. Additionally, engine noise levels and smoothness of operation are night and day difference with / without preheat, clearly indicating much less wear from the free flowing oil. IMHO, preheat at less than 40 deg F is well worth the cost of electricity to save cold start wear and tear on all engine components.