The flashpoint of a liquid is the lowest temperature at which it will generate sufficient vapor to ignite when it is exposed to a source of ignition. It is the temperature at which the vapors coming off of oil catch fire. Flashpoint differs from auto ignition temperature, which is the minimum temperature to ignite a gas or vapor without a spark being present.
Safeopedia Explains Flashpoint
In some cases the expected flashpoint may not ignite the liquid. This may be due to the inclusion of a contaminant in the oil. Often this contaminant is fuel, though it may also be such things as cleaner solvents and water. The U.S. Post Office carefully monitor the flashpoint of samples that are sent through their system, as they do not want samples of oil to explode in transit. In the case of the majority of gasoline oils, the flashpoint is 380 degrees F. For the majority of diesels, it is 410 degrees F.