Which gasoline company makes fuel better for your car?

I would strongly recommend Chevron

Additives are really the only difference. The mechanics I know swear by Chevron's additives and don't recommend Shell's new V-power. I heard this same recommendation from a few different mechanic shops after I had a bout of problems with the fuel system on my car.

Basically, all companies start with the same base mix and add their own little dash of chemicals to create a difference between the brands.

Gasoline is delivered to local mixing tanks through a central nationwide pipe network, so the gas you fill up on at 7-11 is likely from the same pipe as the Chevron across the street, but you get the advantage of Chrevron's engine cleaning agents whereas the cheaper fuel at 7-11 may have the state-minimum of everything.

I understand that it is a good practice to use the same brand of gasoline in your car when possible, and to always stay with the same grade (don't mix premium and regular unleaded, for instance).

If you have the choice, I would always choose Chevron. It is rarely much more expensive, and I find it interesting that Dodge, Chevrolet, and Ford all use Chevron gasoline when testing new vehicles for emissions regulations (even though all three companies have to have the Chevron gas shipped almost 500 miles as there are no Chevron stations in Michigan).

The watered down gasoline rumor seems to be something that every town has - in reality, gas and water don't mix, and so the water would usually not get pumped out of the gas station's tank if someone did attempt to mix them. (This is what the mechanics explained to me.)