Hello, I have been buying fuel from Opet (90% same station and full filling) since I bought the car (208 1.6hHdi). I go to work regularly every day (including the market), and my average fuel consumption is clear. I buy fuel from other companies to try once in a while. Bp and PO said it would go further, but it did not, on the contrary, it gives a sense of decrease in traction. Shell is not bad but I make less km. I bought diesel only from another place (I do not want to specify the company) I was on the road. Since the engine was euro5, I chose fuel and I proved it by living that day: D
I would like to mention that the station where the fuel you buy is not the main thing, the difference in the station change is the traction and range. The car, which constantly receives fuel from Shell, also makes the station changes.
But you can notice the real idea by trying. I don't think you will have a problem as long as you prefer certain brands and stations.