For the 6 months I've owned my Leon its ran on BP Ultimate. Just recently I started using Shell Extra and noticed that the car felt smoother and noticably quieter both on the road and on idle, but the car certainly smooked alot more. No notacible difference in power exept the BP felt had the edge at the top end, (but its not often it sits there). The last 2 full tanks have been V power after about 200miles the car did again feel smoother and more responsive and better at top end than ever before. I've noticed alot less smoke as well. My experiance shows that when I went from Ultimate to Extra I gained about 25miles more than before per tank. I now await to seel how many miles I cna get before the light comes on with V power. Normally Ultimate was around 500miles per tank and Extra was about 520miles. I certainly would keep using it due to alot less smokey, but I now have just got my necter card and I'll be going back to the BP me thinks for the points.
I do vent my tank when filling and from the light just coming on I seam to get about £57 of diesel in when venting no matter where I go, but the different pumps fill up at different rates its hard to tell if your filling up with the same amount everytime. I cetainly feel that the V power gurgles alot less when fill reducing the amount of venting time are required