Not sure if you can add anything after it gels up, if you pour something into the tank I think it would just sit on top of the fuel unless you could somehow stir it in. Never owned a diesel, but I knew a couple of people with them at Ft Drum and they just accepted the fact that anti gel additive was part of their fuel expense in the winter and added it to every tank. Here in Denver I think you would want to add it in the winter if you are leaving it at the airport just as insurance since the forecast could say 50* the next few days then on day 2 of your trip the polar vortex changes everything.