Further to Gordon's post, people should take oil company's claims about their gasoline being "best" with a grain of salt.

You see, a refinery is very expensive to build, and the cost of pipelining or trucking oil to a refinery is tiny compared to the cost of building refineries.

So, typically, refineries will be built in major cities where there are surrounding oil fields, and the oil from everyone's oil fields will end up in the same refinery... getting there either by buried pipeline or by truck.

Oil companies will "unitize" oil fields that extend under land where the royalty rights are owned by different people or companies so as to maximize the production of oil from that field. That is, if each company knows that every barrel of oil they don't produce from their land will be produced from their neighbor's well on their neighbor's land, they'll all produce as much oil from every well they have as quickly as they can to prevent losing oil to their neighbors. That results in less oil recovery from the reservoir than would happen if they "unitized" the oil reservoir and produced oil from it in a more sensible way (such as by injecting water at some wells to maintain the reservoir pressure, and thereby maintain high oil production from other wells, for example). Similarily, they can re-inject the solution gas that comes out of the oil to maintain the reservoir pressure and produce more oil out of that reservoir than would be realized if they each went nutz producing as much as they could from their own wells.

So, while you may have an Exxon refinery in one city, there'll be oil from many different oil companies all going to that same refinery, and each oil company gets a share of the production from that refinery, depending on how much oil they deliver to the refinery on a monthly basis.

So, when you hear oil companies advertising that their gasoline is best, you have to ask yourself how the refinery knows who's oil is coming out of the pipe at any given time. Since everyone's oil is mixed together in the feedstock going in to the refinery, there's no way of changing the formulation of one company's oil or gasoline without changing it for everyone. All of the oil goes in at one end, and the gasoline, aviation fuel, heating oil and asphalt comes out the other end, and it's divided up according to how much oil each company delivered to the refinery, and everyone's gasoline is the same, just as everyone's aviation fuel and home heating oil is the same. The refinery will make sure that it's operation is set up to maximize gasoline production (usually) or perhaps aviation fuel or home heating oil depending on what's most needed and fetches the highest price at the time. But, they can't change the formulation of one company's gasoline without changing it for every oil company that's delivering oil to that refinery.

So, if you live in a city with a refinery, or near a city with a refinery, then almost certainly the gasoline you buy will come from that refinery and it'll be the same stuff regardless of who's selling it; whether it's at a Shell, Texaco, Amoco or Chevron gas station.

I don't know if oil companies formulate their engine oils differently, but I expect there would be differences in synthetic engine oils since those don't come out of a refinery (so far as I know).