Fuel additives
Diesel fuel has all the proper ingredients required to function properly in the diesel engines of today. There is a move to lower the sulfur. You can rest assured that the makers of fuel will not reduce the lubricating qualities without introducing another element to take its place. That would be tantamout to Firestone omitting one step in the tire fabrication process. My preference is no additives. This requires keeping the fuel clean and the tank water free. The biggest enemy of fuel (and diesel engines) is water. Keep the tank water free and you will have no algae, the usual cause of fuel discoloration. Proper filtration will usually protect the engine from fuel contamination. Careful observation when changing filters will warn of the start of tank contamination. That is the time to attack the source. Water seperators are good to have and should serve as an alarm device. Don't forget to check the O ring on the fill cap. Any cracking at all will let water get into the tank.
