Geezer pretty much has it right. Do you think that say, Shell or Chervon puts a load of gas into the pipeline and waits for the same stuff to come out the other end? Or that in most cases the gasoline goes from the refinery directly into a tanker and is driven to your local service station? Doesn't work that way. Instead time on the pipelines are leased. It's the same for natural gas and crude.
A particular company's fuel additives are blended in as the tanker is filled at the pipeline terminal, which is usually within half a day's drive from the filling station. Until that happens it's all the same stuff. Now if you want to compare additives that's a different story. I won't go into that but if you use any of the "Top Tier" gasolines you'll be fine: http://tinyurl.com/r3rqa
A particular company's fuel additives are blended in as the tanker is filled at the pipeline terminal, which is usually within half a day's drive from the filling station. Until that happens it's all the same stuff. Now if you want to compare additives that's a different story. I won't go into that but if you use any of the "Top Tier" gasolines you'll be fine: http://tinyurl.com/r3rqa