Gas oils (petroleum), heavy vacuum; Heavy fuel oil (A complex combination of hydrocarbons produced by the vacuum distillation of the residuum from atmospheric distillation of crude oil. It consists of hydrocarbons having carbon numbers predominantly in the range of C 20 through C 50 and boiling in the range of approximately 350 to 600 °C. This stream is likely to contain 5 wt % more of four- to six-membered condensed ring aromatic hydrocarbons.)