Keep in mind that the us dollar has actually STRENGTHENED vs other currencies in the past 12 months, despite its recent slide. So viewed year over year, it take MORE euros to buy oil than dollars. Not disputing your general point but the slide of the dollar is still small and still very recent.
Menzie Chinn at Econbrowser has a nice pieceon central bank reserves and oil.  It includes this graph of global bank reserve increases lately:

Clearly, the oil exporters are becoming a growing part of the mix, with Asia becoming less important.  However, it's not at all clear that there's any relationship between the decision of what currency to use for trading oil and that of what currency to hold central bank reserves in (a point well made by Jim Hamilton back in January).

Since the Cdn dollar low in 2002, the US dollar is down 47.5%.