Reading the Shell press release, it says you get approximately 1% better mileage with Shell. Woohoo. 1%. The commercial on TV shows 3 Impalas, and the yellow Shell one drives farther. The funny thing I kept on thinking is that it can go 1% farther, and from where those cars were driving, even the Shell Impala with its 1% extra range (what's that, maybe 3 or 4 miles) can't turn around and have enough gas to get back to a gas station since it was driving on some deserted cliffside road.
The shell in my town charges more than everyone else. 1% increase in gas mileage should be a 1 or 2 cent difference, but Shell charges approx 5-7 cents more per gallon than the Mobil or BP.