Some post offices mistake oil samples for hazardous material. Oil is not hazardous material. Blackstone Laboratories' oil sample kit meets all postal requirements. To see a letter you can give the post office to help reassure them, please click here.
The TAN, or Total Acid Number, is a neutralization test that determines the acidity of an oil. In order for the TAN to be useful, you have to know the TAN number on the unused oil product. An increase in this number indicates increasing acidity of the used oil.