We think of oil as being a single substance, but there actually are many different kinds of oil. Kinds of oil differ from each other in their viscosity, volatility, and toxicity.
Viscosity refers to an oil's resistance to flow.
Volatility refers to how quickly the oil evaporates into the air. Toxicity refers to how toxic, or poisonous, the oil is to either people or other organisms. When spilled, the various types of oil can affect the environment differently. They also differ in how hard they are to clean up. Spill responders group oil into four basic types.