http://www.americanprogress.org/issues/2008/05/weak_dollar.html The fall of the dollar has affected oil prices in two specific ways. First, as the dollar falls against the euro and other major currencies, oil-exporting states have been demanding more dollars per barrel of oil to protect their ability to meet expenses paid in euros and other currencies.
This can be most clearly seen in the price of oil (the spot price for Saudi light crude) as measured in U.S. dollars and euros during the first four years of the current Bush administration. As the dollar weakened, the dollar price of oil increased proportionately.
Measured in dollars, oil cost about 28 percent more on average in 2004 than it had cost in 2000, but the price remained relatively constant if measured in euros. In fact, Europeans were actually paying about 8 percent less for oil in 2004 than they had paid in 2000.
More recently, the declining dollar has pushed the price of oil and other commodities higher for a second reason. Retirement funds, hedge funds, speculators, and other institutional investors around the world have tried to protect themselves against further declines in the dollar by moving money into commodity futures that are denominated in dollars—financial instruments that will remain stable or even rise against other currencies even as the dollar falls.