If you use full synthetic lube bumper to bumper, you'll typically gain about 4%. Not great, but I'll take a 4% savings in fuel costs, plus the other benefits of synthetic lube, all the time.
Let me help put that 4% into perspective with actual numbers.
For ~24,200 miles over an 18 month period, I tracked my Tahoe's fuel (mileage, useage and costs) online. Bone stock 4wd '07 LTZ, because after performance mods on an '09 135I 6mt, nothing could make the Tahoe compare.
During that 18 month period I spent ~$4,710 on fuel for an average MPG of 14.6. I actually ran synthetic fluids, changed my engine oil about every 5K, and serviced my transmission, both diffs, and transfer case once each during that time. With that in mind and assuming that the 4% is a real number and supported by science, I would have had a total fuel cost of ~4,906 for that 18 month period. In other words I had a total fuel cost savings of ~$196.
I just guessing, but I would bet that I spent more on the synth oils than they allowed me to save in fuel costs. Arguably, I received other benefits from the synthetic oils that offset the extra cost.