Fuel oil is a broad term that could refer to a number of different products ranging in density from kerosene to Resid. However, it typically refers to residual fuel oil or No 6 fuel oil.

Grades of fuel oil used in US markets

No 1 - A fuel oil comprised of kerosene range material. Also called stove oil or range oil.

No 2 - A fuel oil comprised of light atmospheric gasoil material. This is the fuel oil used as a home heating oil. It is very similar to diesel, but of lower quality. Typically it has a lower cetane and higher sulfur content. Also called Bunker A.

No 3 - This is a now obsolete light gasoil range fuel oil, that has been combined with No 2.

No 4 - A fuel oil comprised of VGO range material, intended for commercial heating furnaces lacking a preheater.

No 5 - A fuel oil comprised of vac resid range material with a low viscosity, allowing it to be pumped without preheating. Also called Bunker B.

No 6 - A fuel oil comprised of vac resid range material with a high viscosity, requiring preheating before pumping. Also called residual fuel oil or Bunker C.