I emailed BlackStone and they sent me a nice, official letter and instructions on how to handle the USPS. The letter details what I am shipping, how it is sealed and that motor oil is not hazardous material. In fact, the MSDS clean up is soap and water.
Partial quote: "... The product being sent in our oil sample container is new or used lubricating oil. It is in no way hazardous, flammable, or toxic. The average flashpoint of used engine oil is 380°F. Used transmission fluid or lube oil has an average flashpoint of 340°F. The minimum flashpoint for all oils we receive is 250°F. The postal regulations for Hazardous Material (DMM Issue 56 C023. 4. 3) indicate that:
“For air or surface transportation without restriction, if the flashpoint is above 200°F (93°C). Mailable combustible liquids must be properly and securely packaged to prevent leakage. ” "
Way to go Blackstone.