I've had fuel gelling issues in the past, when it gets to 0 degrees F or lower. Always at the filter, like everybody else. I had always used Power Service white bottle, and am now convinced that stuff is far from the best, even at high concentrations. My tractor would be hard to start at anything below 5 degrees, and ran rough and surged for several minutes. Last fall, I changed to Hot Shot. Last week we had temps around -7 to -10, and that tractor had to start because we had 15 inches of snow I had to plow. One 10 second cycle on the glow plugs, hit the key, it fired right up in 2 seconds each time. Ran smooth right from the go-get, no issues at all. Hot Shot is the only additive I'll ever use in that tractor in the future. Calls for 1/2 oz per 5 gallons, I had put in a 3/4 oz - 5 gallon concentration on the last previous fill.
Also, be very careful using that red bottle 911 Power Service. It shouldn't be used as a substitute for whatever other additive you're using. It to take care of an immediate gelling problem, not as a long term additive. Read the directions carefully. I think somebody else here mentioned a You Tube video comparing the various diesel fuel additives. There is an excellent one by a guy that calls himself Project Farm. He puts out a lot of videos where he compares various items. He is scientific and impartial, and I recommend people take a look at his stuff. The one on diesel additives is extremely good.