Does the transaction itself have to occur using USD? Or is the current USD equivalent in whatever currency?
I don't believe it is mandatory for all oil to be traded in USD, there are other currencies that are used, such as the euros. Most countries do have to trade in USD, to include all types of other imported goods. I'll have to research on it, but if my memory is right, most of our "axis of evil" list are all countries who attempted to trade oil in currencies other than the USD.