Why would it make any difference at all if oil is priced in Euros or US Dollars? It might make a psychological difference, which would wear off quickly. Would it mean that oil producers would only accept Euros as payment for their oil? No, you are simply changing the way the price is calculated. Right now the dollar weakens and the price of oil goes up. If you price oil in Euros, then the dollar weakens and the price of oil in Euros stays constant while the price of oil in terms of dollars goes up..... same as it does now.
nitro