Interesting I did notice the car ran smoother and quieter this week but that was after the Asda fuel (only added £40 as the pump was so slow in the rain!), would the good stuff sink to the bottom and maybe I was using the last of that.

I'll give it a few full fills and try properly although still not sure the extra cost makes it worth it.

Higher octane petrol can enable a higher output on an engine which is designed to use it - i.e. a high-compression or turbocharged motor with a knock sensor and comprehensively-mapped ignition and fuelling. In these cases the knock sensor will allow more ignition advance or boost pressure.

It's not as clear-cut with diesel - Shell's own literature for both VPN+ and FuelSave state the same typical 'from the pump' cetane levels. The VPN+ is supposed to have a % of gas-to-liquid (GTL) which is purer than conventional refined fuel, but I don't think they state how much any more.

The filling stations are now switching from winter to summer diesel, which can make a difference too.

The Shell Club give cash back on V+ which helps with the cost a bit and combined with the mpg increase all helps out. But I like it because my engines runs better on it.

I haven't checked out the XC60 but if it's like the XC90 it has a tank either side of the rear prop shaft and they can cause the fuel to end up in separate tanks and I'm not sure how they are linked. You may have a non AWD car though in which case that wont apply.

Thanks for the feedback.

Looks like I've got some experimenting to do :)