As engines break in they will get slightly better gas milage regardless of the oil. After 3000 mi I switched to mobil 1 20-50 for the oilhead and the engine was way smoother and looks like I got about 0.2 mpg better milage. The range wasn't as good as a gain as the engine reduction in vibration and harshness.
Also you can practically double the milage before you need to change the oil and that makes up the difference in price.
I only run full synthentic now.