The petroleum that flows from a well in the form of crude oil comes in many varieties and types, ranging from light-colored oils containing mostly small hydrocarbon molecular chains to black, nearly solid asphalt-like large hydrocarbon chains.
These crude oils are very complex mixtures containing a plethora of different compounds made of hydrogen and carbon. These compounds (known as hydrocarbons) can range in size from methane (containing one carbon and four hydrogen atoms) to massive structures with 60 or more carbon atoms. This molecular size distribution can be used to our advantage. Most lubricating oils come from petroleum or crude oil. In order to get lubricating oil from a crude oil, the crude oil must be sent through a refinery.
The refinery takes from the crude oil a lot of molecules of various sizes and structures that can be used for different things. For example, gasoline, diesel and kerosene are all derived from crude oil. Lubricating oil relates to hydrocarbon molecules of a particular size (in the range from 26 to 40 carbons). Fairly large and heavy molecules are needed to work as lubricating oils.
The molecules that are used with gasoline and kerosene are smaller and have fewer carbons in the structure of the molecule. The refinery puts these molecules in little silos based on size and weight, and removes impurities, enabling each of the products from the crude oil to be utilized.