Medium crude oils are the most common type of crude oil. They are more viscous than light crude, with an API gravity between 22.3° and 31.1° API. However, they are less volatile than light crude oils and are therefore less susceptible to evaporation.
Medium crude oils typically have a blend of light oils and heavy oils. Distillation can be used to separate out the more desirable light hydrocarbons. The remaining heavier oil components, called “residuum” after the refining process, can undergo further processing to yield other valuable end products, or they may be sold as-is for lower profit than the light hydrocarbon components.