Hey Robbie, I'm fairly sure that in Alaska or very far North, kerosene or jetfuel is used, which is far less viscous. In fact, special injector pumps are available from Navistar that have special hardened parts to withstand the increased abrasion of the thinner fuels.
400 gls of fuel will get an 18 whlr upwards of 2800 to 3200 miles assuming fantastic mileage, light load, no headwinds or pavement covered in water or snow, etc. The uncle doesn' add anything probably cause he has fuel heaters.
However, even with heaters, figure he fills up in Fla. in January and goes to Bangor or or Duluth with a temp difference of maybe 100 degrees, might have a problem with that warm weather no. 2 from Fla.
My company operates 50 states. In the 48, we use fuel locally delivered and the units are fueled everytime they arrive at a fueling terminal when the trip is concluded. So, the fuel is "changed" frequently a is pretty much local stuff. In other words, a unit that moves from NY to Portland will stay in the Northern zone and be fueled with "treated" or cold formula fuel the entire trip in winter.
Ken2 and sub zero both answered the question. The fuel is not changed "chemically" except the viscosity is thinned much the same way engine oil for colder climes is made less viscous.
That is why jets don't use No.2. It would get them off the ground more efficently but it would be a problem at angels 38. Heavy jet a/c years ago used a water injection system for takeoffs to boost thrust from the jp4. I don't know which has been improved, the fuel or the engines or both.
