I'm currently running 10W40 Castrol-brand, semi-synthetic and getting better mpg in warm weather than I got when I had 10W30 Amsoil, full synthetic in it. But one thing has changed that seems to have a much greater effect than oil viscosity and that's a different brand of tires; going from Bridgestone Battlax's to Michelin P4s; the latter creates a more optimistic trip meter error, but even after accounting for that error, I'm getting one or two more mpg for commutes and at least two better for trips.
My fantastic mpg numbers in a mid-size parallel twin has me excited, and I'd like to take it further to see the potential of this bike while maintaining practicality and maintaining a basic stock-ness to the bike, but I'm extremely limited on resources and have a limited skill set for constructing things even if I can figure out what to construct. But I'm thinking that if I could produce and mount a proper tail, it would not only make my current bike more fuel frugal, but would also make it more usable...That is, except for regular chain maintenance via a rear wheel lift.
I'm thinking that oil is one of those finishing touches adjustments that can help but somewhat limited, and that anyone looking to hypermile or increase mpg of a motorcycle should look at things like tires, riding style, windshield placement and mount, helmet and riding gear, placement and use of accessories or lack thereof, and adding a tail that can reduce drag w/o drastically impairing stability, which is not an easy thing according to the article linked below.
Tony Foale Designs, article on motorcycle aerodynamics.