Another concern for diesel users is the cold flow rating of the fuel. In the winter months, distributors of diesel fuels use additives or blend the fuel to enhance the fuel so it will not clog filters under very low temperature conditions. At very low temperatures, waxes, that are naturally present in all diesel fuels, may crystallize and plug filters. The additives or blending agents keep the waxes in solution. On rare occasion, gasoline and diesel fuel may gets contaminated.
Water is the most common form of contamination observed in gasoline.
Water may enter underground storage tanks from many sources, and it does not mix with gasoline. Water sinks to the bottom of the storage tank and may get pumped with the gasoline if the water levels get too high. Retailers are required to limit the water levels in the storage tank to prevent this. On rare occasions, gasoline and diesel fuel may get mixed together in the distribution system. The resulting fuel may not suitable for use in either type of engine.