This has been gone over before. The petroleum imported into the United States comes from these sources (thousands of barrels per day, Nov. 2008):

Canada- 2028 crude, 2532 total including refined.
Saudi Arabia 1487, 1514 total
Mexico 1296, 1406
Venezuela 1080, 1236
Nigeria 775, 827
Iraq 476, 476
Angola 450, 450
Algeria 381, 677
(smaller quantities from other sources)

Oil refined in refineries owned by any one company comes from whatever sources are convenient to their location, and what is sold at particular service stations depends upon what is available from refineries in that area- e. g. I buy at a Shell service station, but the gasoline comes from a refinery operated by Exxon-Mobil.

To say that one's purchases benefit one particular country is nonsense. Venezuelan crude may be strongly represented at stations near where the crude is landed, but not necessarily. Once petroleum enters the pipeline system, its destination depends on contracts and not source.

Incidentally, more and more petroleum is being exported from the source country in the refined state, as super refineries are constructed near the source.