Crude oil. Image 2 of 2. Crude oil dripping from a glass stick. This illustrates the viscosity of crude oil. Viscosity is the internal resistance of a fluid to the flow of that fluid. See T110/584 for the formation of this oil drop. Crude oil is oil from the ground before it has been refined. It contains a varied mixture of hydrocarbons that are refined (separated) by heating. This process produces parts called fractions. Fractions that are obtained from crude oil include bitumen, fuel oil, lubricating oils, diesel, jet fuel, petrol, and petroleum gases. The fractions are used as heating, aircraft, and motor vehicle fuels, as industrial lubricants, and to make products such as plastics in the chemical industry.