I do ok with synthetic oil in it until about 28*f. Last winter I used a metal pail and the smallest aquarium heater they had at wally world. If I was going to be in the shop for a few days work I would leave the cheapo heater on. If not I would pull it fearing the ice would crush it.
sometimes my problem was slush in the impeller. Generally once you get to machining the heat pulled from the operation warms the coolant right up.