American Petroleum Institute gravity is a standard developed to measure how heavy petroleum is compared to water, in relative density. If the API is greater than 10, it is lighter than water and floats. If it is less than 10, it is denser and sinks.
The higher the API degree, the denser and more valuable it is. However, petroleum with an API of greater than 45 have molecular chains that are too short to be of value. Petroleum with an API of 40-45 have the greatest value.