I ran more than a hundred fifty thousand gallons through my aluminum tanks on my truck, and the inside, including the bottom, was as clean and shiny as the day it was made. The idea that “crude” in tanks, or tanker trucks, is common is another myth. Adding water to tanks to increase the volume is also a myth, the water will separate very quickly, so when the level got low you would be pumping almost all water and any car filling up wouldn’t go very far. Anyone trying this would be detected very quickly. There may be a few isolated exceptions, but in almost all areas gasoline comes from a very few, if not one, local supply point. Any additives are added to the tank truck in most cases. Storage tanks do accumulate water, mostly through condensation, but they are checked periodically and any significant water is removed. The same thing happens in your vehicles tank and it’s best not to leave the tank below half full for long periods. The more empty space in your tank, the more tank surface for condensation to form.
Frequently going from warm areas to cold areas and vice-versa can also increase condensation buildup, for instance keeping your car in a warm garage at home and parking outside at work in cold weather. Frequent short trips, which is common to most cars, also adds to the problem.