I ship full cans all the time...but I package them very securely and in a plastic bag.
JimQuote off an Ebay discussion board:
Here all that matters is where the flashpoint falls in the USPS/DOT defined ranges.
According to Google, flashpoint of motor oil is 420-485 deg F.
Per USPS [ http://pe.usps.com/text/pub52/pub52c3_020.htm
Note: A liquid with a flashpoint above 200° F (93° C) is not regulated as a hazardous
material and may be mailed subject to the general packaging requirements in DMM 601.1-7,
provided it possesses no characteristics of another hazard class.
Above 200°F (93°C): The material is not regulated as a hazardous material.
Such nonregulated materials must be properly and securely packaged to prevent
leakage under the general packaging requirements in DMM 601.2.0.
Motor oils appear to (potentially) only fall into hazard class 3, and their high flashpoints make them neither flammable nor combustible by the definitions (which then kicks them out of that class.)