Ultra Low Sulfur Diesel Fuel is here Starting June 1st, 2006, refineries in the United States are required to produce a minimum of 80% of their highway diesel fuel as Ultra Low Sulfur Diesel Fuel (ULSD) – having a sulfur content of no more than 15 ppm. The remainder may continue as Low Sulfur Diesel (LSD), limited to 500 ppm sulfur content. In California, 100% of all highway and off-road diesel fuel will be ULSD. The process of reducing sulfur, or hydrotreating, decreases the inherent lubricity of the treated fuel, and that there is a corresponding relationship between the degree of hydrotreating required and the loss of lubricity (i.e., the greater the degree of hydrotreating required, the greater was the loss of lubricity). Operating engines with reduced lubricity fuel results in premature wear to some fuel systems – with rotary fuel systems being the most susceptible to damage.