I just use rocket fuel.
But seriously, it has a negligible effect on gas mileage. Higher octane gas prevent engine knocking from early fuel ignition, which can fuck up your engine.
It was recommended to use a higher octane fuel once when I had a fuel pump replaced on my old Grand Am, but other than that, use what the manual says. There's really no benefit to using a higher octane gas in a car that is specified for regular.
Remember...remember us. Remember...that we once lived...
The artist (formerly?) known as Citrik