No oil will change fuel economy in any noteable way, you might see something like a 2% difference averaged across a large fleet switching from 5w-30 to 0w-20, When individually fuel economy changes it has more to do with the difference between winter and summer gas and whether you're running the AC less, changes in driving habits, etc, changing between different oils will have virtually no distinguishable difference in fuel economy at the individual tank level assuming all other variables are the same.