Running the tank low has nothing to do with getting bad fuel. You will cop water/dirty fuel at any stage as the fuel pick up in the tank sits at the bottom. And as water is heavier than fuel, it will always sit on the bottom. Same as your cars fuel tank. Dont put blame on running your tank low when you pick up water and dirt. Running the tank low has nothing to do with it.
Terry, most of the time getting crook fuel usually comes from the tanker. You get lots of cross contamination from these tankers. They dont always clean the tanks out after each trip so in the same tank on the truck, you might get diesel one trip, then 98 the next and then 92 another trip etc. So its not uncommon to find traces of diesel and other contaminants in your 98.