Sulfur is not a
lubricant in of itself, but it can combine with the
nickel content in many metal alloys to form a low melting point
eutectic alloy that can increase lubricity. The process used to reduce the sulfur also reduces the fuel's lubricating properties. Lubricity is a measure of the fuel's ability to lubricate and protect the various parts of the engine's fuel injection system from wear. The processing required to reduce sulfur to 15 ppm also removes naturally-occurring lubricity agents in diesel fuel. To manage this change ASTM International (formerly the American Society for Testing and Materials) adopted the lubricity specification defined in ASTM D975
[12] for all diesel fuels and this standard went into effect January 1, 2005.
[13] The D975 standard defines two USLD standards, Grade No. 2-D S15 (regular ULSD) and Grade No. 1-D S15 (a higher volatility fuel with a lower gelling temperature than regular ULSD
fuel manufacturers have been adding a lubricating agent to their fuels for years.