I'm a hateful engine geek and did quite a few tests on fuels with my 500e.
Essentially I had no performance improvement with regards to the butt dyno. I used to get a smoother hot idle from both Tesco 99 and V-Power and before that Optimax.
What I did see was a regular drop in mileage from 550 to 500km from a tank over many many tankfuls. I average pretty much bang on 21mpg when using V-Power and was pretty much exactly 10% less on Tesco 99. This was over about 5 tanks of each when I was doing the back to back testing, yes I was single at the time
Now my 110 Defender which currently has 218,000 miles on it, it is running well as i'm a diesel guy by trade and I only get minimal smoke when running into the governor or when off boost on a cold engine. I have run it on a 50/50 mix of Rapessed oil and diesel, Extreme winter fuel in the north of norway down to -36 Degrees, 5 litres of old engine oil after filtering and i've even run it with 5 litres of EP90 in the tank. This was all added to a nearly full tank of diesel. On all these mixes and once warm the engine never smoked. When I fill with Morrisons diesel though it will start smoking after just a couple of miles. Time and time again so now I never touch Morrisons diesel.
I did run a tank of BP Ultimate diesel when I first picked up the Defender which seemed to improve it but I think this was mainly down to giving it a long run at speed (Well 65mph for an old Defender counts as speed
) more than the fuel itself.
There was a post on Pistonheads a long while back and one of the guys who designs the mixing system for the fuel. He explained very well how the base fuel is the same but the amount and quality of the additives changes quite a lot between tanker fills depending on who wants it.
Personally I fill up the Merc with V-Power and the Defender with whichever branded fuel station i'm closest to.
I think getting fuel from a station that needs regular fresh deliveries is also important as it can go off quite quickly.
Dave!