Residual fuel oil is one of the lowest-value petroleum products from a refinery. It is essentially a byproduct of producing the light products that are the primary focus of a refinery.

The primary end use for residual fuel oil is as a fuel in simple furnaces such as power plants and industrial boilers. It is also the primary fuel used on ocean-going ships, where it is called bunker fuel.

Nonetheless, residual fuel oil has some quality specifications that it must meet, for performance and environmental reasons. The most important are:

viscosity - a measure of a fluid's tendency to resist flow. Lower viscosity is more desirable

sulfur content - fuel oil has a maximum sulfur content determined by environmental concerns. This is typically set as a maximum sulfur content in wt %.

Residual fuel oil is the heaviest among a range of different fuel oil grades.