Saudi services to US government

"From 1991 to 2003, Saudi Arabia was the top supplier of foreign crude to U.S. refiners, a position it maintained as a way of supporting its political alliance with the United States. To keep the flow of Saudi crude to the U.S. high amid growing competitions from countries like Mexico, Saudi Aramco, Saudi Arabia’s national oil company, put a lower price on the crude it sold to the U.S. than to other countries. ...To judge from the political contributions they made, the refiners appreciated the gesture." (thanks Khelil)