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You have heard all the stories about contaminated fuels. No.2 diesel fuel is one of today's most variable fuel sources. Its quality and purity range unpredictably from very good to very poor. Current API specifications allow "acceptable" levels of impurities (sulfur and wax) and contaminants(dirt,ash, and water).
It's likely that these regulations will weaken even further in the future. The deterioration of fuel is almost inevitable. Contaminants are introduced into the fuel system through mixing, transferring, and storage.