old_man and Lightfoot; I believe some will say there is a thermostat correction in order, or anti-freeze is dying, and not to block radiator like what was customary in times past. I'll leave that to the experts.
In Alaska, when the fuel mix is changed for winter.., come summer, (we only have a few weeks of spring, and fall), the fuel tanks often have ice cubes, or slabs of ice banging around until they melt away. What I do, as many others, is to get a 'turkey baster', and a gallon can of acetone from the hardware store. Before fueling, I introduce a couple of ounces of that, and then mix it all up with high octane. In layman terms; Dries out any water, and I believe it helps to atomize the fuel, have no icebergs, and engine runs 'mo-'betta. Usually get an extra mile, or two per gallon of fuel, and a bit more in the summer. I've used it in a '74 Chev., '79 J.Cherokee, 2006 Kia, and currently in my '89XJ. Has not been deteriating any fuel 'O'-ring seals, rubber hoses, etc., to any vehicle I've been running.
So far this year, my routine at minus 10 degrees f., is to separate the electrical plug from an oil pan 250watt Moroso heating pad. After idling for about three minutes, it takes my XJ about 3 miles to get heat out of the dash vents. XJ won't go below 16mpg at this minus temp., and rarely gets 20 in the summer, (sans the Moroso).
I think it's worth the extra pocket change to get the highest octane possible from the pumps in order to insure a complete burn in extreme cold. LOL.., I've other major worries. Moose, drunks, cops, slippery roads, and half way think something is going to break! Also, I try not go faster than 45mph, (which also helps the mileage figures), but sometimes have to go to 50, 55, on a 50 mile two lane road.