::::If there is a credible link to a US Gov't definition for the capacity of a tradable barrel of oil, then it should be referenced in the main article.  As the article states, there are several definitions for the capacity for a barrel (42 or 55 gallons).  I highly doubt that the US gov't is or was responsible for defining the capacity of a barrel of crude oil used for commerce (or that it is a world-wide legal requirement to use a US-gov't definition) but if there is any such legal requirement then again a reference should be stated in the main article.  To say that NYMEX has "no legal right to define measurement units" is a bogus argument.  What NYMEX has is the right to define or specify units that are pertinent to trade.  If NYMEX states that oil is to be traded in terms of barrels, then it is not "illegal" for NYMEX to state the capacity of this thing called a "barrel" in terms of unambiguous STANDARD units, or to point to an authoritative source (authoritative as far as NYMEX is concerned) that defines the capacity of a barrel.