But for conversations sake, if we're talking about diesel trucks, and specifically an old mechanical diesel like a 7.3 ford or a 12 valve cummins, and we're talking about stored diesel, ideally a class D died diesel with a high sulfer content, then you'll get considerably more lifetime out of it than the recommended 18-24 months.
Additionally, you can burn about anything, old motor oil, fry oil, diesel with algee in it if you treat it and change your filters. A good, mechanical, not very picky diesel will burn fuel that is 20+ years old. Not forever, and you'll go through some filters and maybe injectors. By the time you get to fuel sources that can't be burned in a machine like that, or you've burned it up past your parts and ability to repair. you'll probably have a lot of other problems.
If we're talking generators, you can get off the shelf propane generators and propane stores well.