I think a lot depends on what wattage rating pad you have. I have a 250 watt on a 5 quart sump (plus one more quart in the filter). I leave it plugged in all the time, but have no fear of burning the oil if I plug it in cold (but I am never much below zero F). I started it up one cold day and then went under the truck and felt the pan (not by the pad--ouch) and could feel cold oil coming back into the pan. What happens is the truck fires up nicely in cold weather when plugged in, but the oil gets cooled going through the engine. Now somewhere that will equalize and I doubt the oil gets near as cold as if you never had the pad on. Regardless, I really like having the pad and think it helps a lot. I know it starts a lot harder on cold days leaving work where it sat 9 hours unplugged. You might want to search. There was a thread or two on this site regarding the heating pads (not sure which forum) and Wolverine became a site sponsor (hopefully still is).