The majority of motor oils are not flammable. However, all motor oils are combustible and should be handled with caution around heat sources of any type.
Flammability in regards to liquids is determined by its flash point, the lowest temperature at which it can release ignitable fumes into the air. Any volatile liquid with a flash point lower than 100 degrees Fahrenheit is considered flammable. Any volatile liquid with a flash point higher than 100 degrees Fahrenheit is considered combustible. Although a handful of motor oils designed for use with motorcycles are flammable, the flash point of most motor oils is at or above 400 degrees Fahrenheit, classifying them as combustible.