I don't know what's going on with this. I have a 98 mustang and was using shell for awhile but I found myself having to fill up every two days after going only about 120 miles on half a tank. I would also watch the gas gauge just slowly drop while driving since the moment after fill up. The guy I bought the car from said he only used Chevron in the car since he got it so I started using Chevron. I'm getting way better gas mileage and can last about 180 miles on half a tank. Filling up about every five days and I haven't changed the way or where I've driven the car. Does anyone else have this issue or know what it could be? It kind of sucks to only be able to use chevron or waste money at another gas station which will only get me around the corner.