Makes sod all difference that I can see. MPGs will always change unless you drive the exact same route in the exact same traffic/weather conditions with the exact same amount of air in your tyres etc etc etc, which lets face it, never going to happen so you can never test it properly.
Shouldn't 8vs be run on 95? I seem to remember seeing a sticker on the fuel flap but can't quite remember what it says.
I use shell normal fuel, as my OH has an Egg credit card that gives us cashback at shell stations, and the shell is competatively priced to start with.