Technically, the flash point is the lowest temperature at which a liquid will generate sufficient vapor to flash (ignite) when exposed to a source of ignition, or fire. In other words, at what temperature do the vapors coming off your oil catch fire? For most gasoline oil samples, it's around 380F. For most diesel samples, it's around 410F.

When the flash point reads lower than the "should be" value, it shows a contaminant in the oil. Most often that contaminant is fuel, though other things can affect the flash point too, such as solvents (like engine cleaner additives) or water.

The Post Office is particularly concerned with the flash point of your sample because they want to make sure that your oil isn't going to explode or catch fire while it's going through the mail. We have reassured them that's not going to happen, though sometimes they still can be grumpy about taking oil samples. You might be better off sending it UPS or FedEx.