As an alternate view to Charon's proferred article:
A third OPEC country which has recently fallen out with the US government is Venezuela and it too has been showing disloyalty to the dollar. Under Hugo Chavez's rule, Venezuela has established barter deals for trading its oil with 12 Latin American countries as well as Cuba. This means that the US is missing out on its usual subsidy and might help explain the American wish to see the back of Chavez. At the OPEC summit in September 2000, Chavez delivered to the OPEC heads of state the report of the 'International Seminar on the Future of Energy', a conference called by Chavez earlier that year to examine the future supplies of both fossil and renewable energies. One of the two key recommendations of the report was that 'OPEC take advantage of high-tech electronic barter and bi-lateral exchanges of its oil with its developing country customers', i.e. OPEC should avoid using both the dollar and the euro for many transactions.
Cóilín Nunan: Oil, Currency and the War on Iraq Well it isn't in an retail oil/petrol trade magazine that's entirely funded by advertising from the oil & petroleum industry (stop me if I'm wrong). Furthermore the Citgo article has the stench of a advert deal and an editorial initiated by sales.

. I see Citgo is a wholly owned subsidiary of Petroleos de Venezuela S.A., the company whose entire board was sacked by Chavez in January 2002 and then mostly reinstated a couple of weeks later, a move that angered it's executives. No sour grapes there, then.
This Cóilín Nunan guy of course has his own anti-corporate greenie axe to grind, but somewhere between the two articles we might just find a glimmer of the truth.
That said, US policy in Latin America seems to have long held the belief that democracy is a bad idea. Possibly because those democratic governments have a nasty habit of nationalising stuff that a lot of multinational corporations want.