FWIW we just took a 1700 mile trip to Canada and back. I used 87 E10 for the trip (** we left the house on 88 E15) and my mileage is listed below. All values are from my Fuelly app so hand calculated. We traveled a total of 1703 miles, spent $302.76 and used 69.558 gallons, averaging 24.5MPG. IMO, driving conditions (highway/in town), speed, and weather (temperature and humidity) play a much bigger factor than the oil viscosity.
26.3mpg, 29.0mpg (average 27.1mpg) - to Canada at various speeds but usually between 75 & 80MPH, temps cool at 70 - 75F, low humidity and no wind
22.3mpg, 28.1mpg (average 23.8mpg) - in Canada for various in town and short distance trips
23.8mpg, 20.9mpg, 24.5mpg (average 23.3mpg) - home to Iowa, again various speeds but between 75 & 80MPH, temps hotter at 85 - 90F and humid, mild wind w/ some gusts
just my $0.02