I use Shell V-Power everyday (99+) and I think it's different from the normal fuel not just during summer. Here in Italy, right now, the weather is quite cold and I still prefer the V-Power (also because we don't have the 98, but just 95!!!.
It's more expensive (about 0,2 € per liter), but I never had any problem.

If you want, you can take a look here (just Italian, sorry).

http://www.shell.com/home/Framework?...html&FC4=&FC5=


Bye

Andrea