Oil isn't a uniform thing - it's closer to marble than limestone. People import tuscan marble all the time due to the grain and qualities of the piece itself -

Oil isn't a uniform thing - it's closer to marble than limestone. People import tuscan marble all the time due to the grain and qualities of the piece itself - it's quite the same with oil - not every barrel was created the same and America has some processing facilities specialized to consume a quality of oil not found domestically in large volume.