It is my opinion that oil should be priced in American dollars because America is one of the largest and most influential importers of the product. America's interests in the global economy are largely based on oil, and the price of a product that is this valuable to a nation as rich as America should be valued based on American currency.
Perhaps oil should be priced in dollars as opposed to euros because America is the most powerful country in the world and therefore should have that right. Maybe dollars are more widely known than euros too and should be used for pricing many other things as well, not just oil.
Oil should remain priced in American dollars, simply because it is not worth the effort to change the system. It is a simple enough process to convert from US dollars to Euros. It is done every day for commodities trading, importing and exporting of goods, and by thousands of tourists. It would be far more work to redesign the current system than to continue to perform these simple conversions.
I believe the US dollar is well established as the primary currency to trade oil in and it would dramatically effect the oil markets if they were to switch to the Euro, I believe overall the American dollar is stable and secure enough and transacts enough volume that there is no reason to switch.