The price of crude oil is the main factor in the price of diesel fuel, and fluctuations in the crude oil market greatly influence changes in diesel prices.
The federal requirements now in effect for producing diesel fuel with a far lower sulfur content has clearly led to many changes in the refining process, and with them have come changes in a number of API standards.
U.S. Environmental Protection Agency (EPA) standards now require a major reduction in the sulfur content of diesel fuels and emission levels from diesel engines and vehicles.