Furnace Oil, Light Diesel Oils. Furnace Oil:Furnace oil,fuel consisting mainly of residues from crude-oil distillation. Furnace oil is used for For steam boilers in power plants, aboard ships, and in industrial plants. Commercial fuel oils usually are blended with other petroleum fractions to produce the desired viscosity and flash point. Light Diesel Oils:Light Diesel Oil falls under class C category fuel having flash point above 66C.It is a blend of distillate components and a small amount of residual components.Light Diesel Oils is used for lower RPM engines, Lift irrigation pump sets, DG Sets and as a fuel in certain boilers and furnaces.