In my quest to find "The best oil" for my application I have tried just about every brand of Dino & Syn.
I can tell you without a doubt that switching between all these brands, weights and types of oils, that
none have had any significant change on fuel economy.
I drive a lot of miles for my job and drive mostly the same routes. I also have a Scan-Gauge hooked up to my cars computer to track mileage and it was confirmed by it.
I might have seen 0.5% increase at the most (being generous), but nothing that you would notice in your pocket book or that you could attribute to the oil for sure. This is probably because I keep my engine well maintained and well tuned so things like oil make little difference IMO.
If you want to save money, get a scan-gauge and drive a bit slower and maximize your economy using the gauge to monitor your real-time fuel economy, and look up Hyper Miling on Youtube! Switching to another oil, especially at a higher cost will negate any possible savings you were to gain in fuel economy because you are trading one for the other.
Now if your engine has seen less than stellar maintenance in the past, a good clean up by a good synthetic could result in better mileage but this isn't the oil that is giving you the mileage it's the cleaning process.