No difference. It really is all the same. They regularly buy from each other.
Let's say the closest fuel station to you is a Dinoco station. You'd think they get their fuel from nobody except Dinoco (or Dinoco licensed) refineries, right? Wrong. When they suddenly run low, which happens every Friday, they might call up their competitors from Axl Rose Fuel, GuzzleCorp, FossilLies or even 69 Petro. And they have no problem doing that, because all those companies have called them up when they ran out last Monday.
The fuel business isn't just a greedy business, it's also a really deceptive business. They pretend like they're all competing against each other, when in reality they're all working together. It would be a conspiracy, except none of the fuel companies deny it.