Many will deny this, but it happened to me also. Switched to Q Horsepower recently from Mobil Clean 5000 (Always PureOne filters). I do the exact same trip from Ohio to Canada every 2 weeks. 200 miles, cruise control set at 69mph and 74mph depending on posted speed limit. I use to average 25-26mpg on dino (last 4 trips on dino), now it's been about 27-29mpg with the synthetic (last 4 trips).
Actually experimented with this. All trips had windows up, heat on the exact same setting, outside temps comparable, tire pressure always checked. Wind speeds/direction were also always fairly similar. Filled up before every trip at the exact same gas station (odometer reset to 0), and refilled at the border. Avg mpg's figured out by distance travelled divided by gallons needed to fill to full again (Car has a readout also. It's not as accurate, but also shows improvement)
And, I actually noticed.. On Dino, to maintain 69mph, the engine revved at about 2600 RPMs. On Synth, at 69mph, engine revs at just above 2500 RPMs.
.. Can't be imagining all that!
P.S. 2.7LV6 Sebring