Lets look at a typical motorcycle engine. Say you want to increase your fuel consumption and think that just by changing to synthetic oil you will gain 2 to 3 mpg. Lets look at this Scientifically. I extracted the following out of a Cycle World article:
The friction loss in a typical engine is around 15% of delivered power. Because four stroke engines use half a pound of fuel for every horse-power they develop per hour, a typical mileage of 50 mpg would indicate that highway speeds, we are using 7-8 pounds of fuel per hour, which in turns suggests that the rider is using about 15 mph at the speed.
An improvement of 2-3 mpg in fuel consumption would translate to a savings of about 3/4 of a hp, while the engine's friction loss at that operating point (15 hp) at 15 percent, would be about 2.25 hp. Savings 3/4 of a hp is a 33-percent reduction in friction which is a lot! When I talk with oil chemists and read things they write, they speak of modern low-viscosity lubricants cutting friction by maybe 3 percent as compared with the older 10w-30 and 10w-40 oils.
Thus the big difference between 3 percent and 33 percent reduction in friction makes it appear unlikely.