It most certainly makes a difference in fuel economy. Just one so small you'll likely never notice. It will be there; you cannot avert the laws of physics and those pertaining to pumping losses. However, the variability of daily driving makes it essentially impossible to find the perceptible change in the economy via the vis change, for the average person. The small shift can be magnified by driving a LOT of miles, and/or operating a fleet. Otherwise you'll not be able to discern it.
And I, for one, have never seen any real data that proves that a thicker lube is NEEDED for "severe" service. A good HDEO is a good HDEO, and minor vis grade differences matter not in the overall wear characteristics of a piece of equipment. That, and Ford's near-schizophrenic lube recommendations leave me shaking my head nowadays.