I had a brand new '86 Izuzu Trooper 2 and later bought an '87 Izuzu Pup truck. I went to start the Trooper on a day that was easily -25F plus windchill and could hear the engine rapping like crazy. Dealer eventually put a short block on it after first trying a new head, to no avail, under warranty. I had NO synthetic oil or additives in it. After that nightmare I started using Slick 50 in my 100k+ mile Volvo 240 and my brand new Pup Truck. Never experienced anything similar in either of them and was favorably impressed at how easily they would start in any amount of sub-zero weather.
I also used to sell Red Line Diesel Fuel Catalyst, when I owned a Foreign Auto repair/used car facility prior to '86, when I sold the business.
My shop used it when we would see numerous diesels from out of state that would freeze up on the mountains at the ski resorts when the temps would drop at night. We'd add it to the fuel, fuel filters, and thaw out the cars on the lifts then send them on their way home.
We found the additive reduced cold start issues, burned more of the vapor/water in the fuel tanks and filters/lines etc.
This is just my specific experience, your mileage may vary.
Currently I am not running any additives in my DK-40, but I do keep my fuel and tractor in a heated garage.
I currently run/ran Amsoil Diesel Oil in both my DK-35 and now DK-40. I plan to continue to run synthetic in the DK-40, since I traded the DK-35 for it last year.
Diesel's high compression and fairly frequent oil/filter changes don't phase me so long as I feel I am reducing engine wear by using synthetic oil. For instance, some manufacturers now REQUIRE synthetic oil in their cars, Corvette is one of numerous cars in this category. Good enough for them- then good enough for me.