Seeing as we are now in the 2ed Little Ice Age, I figured I would share what I do to keep my Diesel equipment operating in the northern New England ski area over the winter where my "normal winter" sees temps down to about -25 F and maybe lower if we get a cold snap like we have now. I've operated on and off road Diesels in this environment for about 40 years so I have a good set of practices I follow. By following these, I tend to avoid a lot of issues that I see other operators have.
OK, here we go.
1) Find a good place to buy fuel and keep going there. This is the number 1 step because it's the most important. Ask around or just keep your eyes open for the place in town that the pickup or over the road trucks fuel up at. These people go through a lot of fuel and they quickly find the place that has the fewest issues with water or algae. Stay away from the small store that has that lone Diesel pump that you never see anyone getting fuel from, I have learned that those type setups are an invitation to no end of fuel problems.
2) Pick a winter additive brand and stick with it. I'm serious, pick one and keep using it. All major brands work well enough or they wouldn't still be sold. I'm not going to get into a brand contest here, The brands I have used with good results over the years are: Amsoil, Howe's, Optilube, Power Service and Stanadyne. Personally, I prefer Amsoil, Optilube or Stanadyne but I have used PS and Howe's in a pinch in the trucks. The reason you pick one and use it is that when mixed, they may interact and give you a whole new set of issues to deal with.
3) Treat all fuel for the lowest temps you see yearly. Look, we aren't talking about a large amount of fuel here for most operators, if you treat for winter even in July, if you still have fuel from July in the tractor when that freak cold snap hits in Sept, you won't have low temp issues.
4) Mix the additive at the ratio listed on the bottle, no winging it. Yup, they pay a bunch of chemists and engineers a lot of money to come up with the ratios and they know best. Don't think more is better, I have found that either it costs you more because your using it needlessly or you end up stuck on the side of the road because it didn't work correctly and that costs a lot more.
5) Get a spare set of fuel filters and keep them handy. For a tractor this is easy, keep them on the shelf in the garage. If your in a truck, put them in a ziplock bag with a strap wrench and stow it in the cab somewhere. If you do plug a filter, you have them with you to change out. At some point this will mean the difference between getting home to your bed or sleeping on the road another night. Don't forget to get a new set of spares once you use these...
6) Only buy as much fuel as you will use in 4 to 6 weeks. Remember, the majority of us here are operating a single tractor in a residential setting, these are only going to use 6 to 10 gallons of fuel a month or so, sometimes less. You don't want to be storing 500 gallons for 5 years. By turning over your stored fuel quickly, you assure it's fresh and is blended for the season your operating in.
7) Fill your tank with a funnel with a paint strainer in it. I have been doing this since I bought my first lawnmower from K-mart when I was 12. I used to cut lawns and my dad's friend who was a drag racer told me to always fill through a strainer so that anything in the fuel container wouldn't go into the tank and plug the line. For these Diesel tractors, the paint strainer is even better as if your fuel is clouded, gelled or has algae in it, the strainer will plug solid the instant the fuel hits it. This will save you from mixing in bad fuel with good fuel. If you have a fuel issue you can treat it in the storage containers, not your tractor fuel tank and engine.
8) Store your fuel properly. Do so according to safety and local codes however, keep it in a temp stable location as much as possible and keep the container closed tight. The temp swings and venting are what cause moisture to collect in the fuel and then become issues.
9) Keep your tractor topped up if it sits outside or anywhere with temp swings. See #8, your tractor fuel tank is vented, if it is partially filled, it will expand and contract more allowing more moisture in. Keeping it filled will minimize this. If you park yours in a place that's temp stable this is still a good idea but not critical. I fill mine after every use even though it sits in a garage that is temp stable.
10) Don't resort to old "trucker fixes". There are a lot of these that just won't die. If you have newer equipment, you can't do what the guys 30 or 40 years ago did to keep stuff running in the winter. Things like mixing in gasoline or kerosene to "thin" the fuel, putting gas line "anti freeze", paint thinner etc. These things may have worked on an old GMC 6-71 to get it off the side of the road in 1960 but they will pretty much destroy newer equipment. (They destroyed the old equipment too, it's just that nobody noticed because those old Detroits came apart so often they were used to it.)