When crude oil is extracted from the earth it contains a vast mixture of organiccompounds, which range from very light hydrocarbons to extremely high molecularweight species. In the refinery the crude oil is distilled and various distillation cutsrange from light fractions (gas and light solvents), fuel (gas oil), a lube cut, and theheavy tractions (heavy fuel oil and asphalts). Hot oils come from the lube cut and afterfurther refining the hot oils are selected for viscosity (which partly defines the heattransfer properties) and stability, and are branded and marketed as heat transfer fluids.