As previously mentioned but I thought I would back it up, I used to work in a Sainsburys Petrol Station and alarms bells would ring once the tanks got to 4000 litres but were silenceable at this level. One it hit 2500 litres you couldn't turn them off unless the pumps themselves were turned off, hence pumps being shut off. There isn't one big tank we had about 4 Unleaded tanks, 4 Diesel tanks and 2 Super Unleaded tanks I think it was.

But I now use Shell, mainly for the points admittedly! I only use 95ron in mine so the Optimax/Super Unleaded argument to what's better doesn't apply to me