19 U.S.C. § 1507

(a) In general The Secretary of the Treasury is authorized to prescribe and issue regulations for the ascertainment of tare upon imported merchandise, including the establishment of reasonable and just schedule tares therefor, but (except as otherwise provided in this section) there shall not be any allowance for draft or for impurities, other than excessive moisture and impurities not usually found in or upon such or similar merchandise. (b) Crude oil and petroleum products In ascertaining tare on imports of crude oil, and on imports of petroleum products, allowance shall be made for all detectable moisture and impurities present in, or upon, the imported crude oil or petroleum products.