Oil (variable) = Dollar (variable) <= The value of everything is variable to some extent, but some assets are more reliable than others. For a long time the dollar was about the most stable asset available, so it was easier to deal with. Now when the price of oil goes from $70 a barrel to $75 a barrel you have to figure out if demand for oil went up or if the US Govt pissed away a trillion dollars and the value of the dollar just dropped.