The euro float is certainly large enough to handle the oil trade. If money is purchasing power, the financial institutions of this world can provide plenty of Euros.

The weakness of the Euro is political. Not all countries in the Euro market are happy with membership. See Italy for one example. It would prefer a currency it could more easily inflate (devalue or debase).

Hey...a petrocurrency has to start somewhere.  The Euro already acts as a defacto "basket" currency.  Makes it easier to add additional countries/currencies.  I think it would be quite easy to switch over, if/when countries decide to give it value.  All it takes is "agreed upon" confidence.