Worked for a guy back in the late 80's, out in Colorado, that had us put unleaded gasoline in the tank when it was below 0. It did keep it from gelling, but if the weather warmed up before we ran those tanks out, the motor would rattle a bit. IIRC.........we'd put about 5 gallons of gas in per side, on a 200gal diesel fill up. So.......that would have been about 5% per 100gal tank..............
I was hauling container loads of Miller beer out of the Burlington Northern RR ramp at the time. Started around 4:00AM so I could get to the distributor in C. Springs by their opening time. Cold starts, lots of idling time at the ramp, and at the warehouse in The Springs. Never gelled.
Whether it was good for the engine..................I have absolutely no friggin' idea. It was his truck, his orders. Never could tell if there was any damage in the time I worked for the guy(shrug). That particular truck had a 9spd..........mostly a flat lander truck.
Used to run Coca Cola from the bottler on York St. up over the mountains to Glenwood Springs during the Winter. Never had gelling problems, but it was hard pulls, mixed with long downhills against the Jake. The fuel would remain warm enough on the downhills, even though the heads were cool. Motor didn't like firing up when you came off the Jake, but it was just a momentary thing. Like to froze to death coming down those passes, even with a Winter front. Coolant got too cool for the heater to run right. 12mph uphill.........12mph downhill. Those were the days.... 90% of the trip was in 4th gear

. 350 Cummins, 13spd Road Ranger. And it was a race car compared to the old 290's.
I'd get guys comin' around me going downhill running at around 20-25mph. And sometimes you'd see those idiots off in one of the runaway ramps. Once you break traction, it's a matter of free wheeling to keep it from going over the side when it went into a skid. The older guys used to give me crap for running so conservative.........but I'm still here to talk about it.
You got the front drivers chained up. Coming downhill against the Jake you'd keep an eye on the drivers. If it looked like the drivers were starting to run in reverse, it meant that they'd broken loose, and were skidding. Hit the throttle, and the Johnson Bar at the same time, and the trailer brakes would straighten you out. I got paid by the load, so it was my call on how fast I wanted to make the trip. But I got home every night I ran that damn load. During ski season I'd run up to 3 times a week. The bars did a roaring trade during ski season...........lotta mixed drinks I guess. Coming back empty was where the pucker factor came into play.
About a month after I left that job, the next guy ran the truck into the back of a snowplow out around Limon.