Personal experience here... (see signature for vehicle)
I ran 5w-30 amsoil ASL for 10,000 miles. I have run 0w-30 amsoil SSO for 5700 miles so far. On average with the same driving condition and route, same weather, and same tire pressure. I have only seen a difference .2 mpg increase. I will say however that this oil makes the motor idle smoother than it did before. I would also assume a winter mileage increase would be more noticeable due to the "0" grade.
All in all the SSO is a better oil for me because I like to change my oil once a year with filter changes every 5000 miles, and anyone who knows anything about the 4.7 (Chrysler) knows that it isn't exactly easy on oil. (numerous sludge issues with this engine and dino oil over the years whenever oil changes went much past 3-4000 miles)