If it's near 20, oil pressure seems to take longer to come up than usual, which is harmful on the engine. Good 4 hours is good, but I keep mine plugged in all night.

 

Even when I was in SC, if it got below 40, I would plug it in with the timer to come on about 4 am. But now that I am up here, it gets plugged in as soon as it gets parked in the driveway and stays that way until it gets driven again. I am sure it's probably a little overkill, but it seems to make it a lot easier on the truck.