V Power every time in my F10 M5. Not entirely convinced it makes any difference.
The few times I have put standard 95 fuel in, my MPG has dropped. Not very scientific but that, combined with some clever marketing (Shell points, etc) swings it for me.
In my 18 months, and 18,000 miles, of ownership my car has had one tank of regular unleaded (when we were on holiday on Skye), one tank of Tesco 99 and an awful lot of Shell V Power.
I reckon I've used up a couple of tanks solely by driving out of my way just to find a Shell station