Here in US the lowest recomended (by Volvo) octane is 87. I use 87 octane fuel in my non-turbo '98 S70 all the time with absolutely no ill effects (I have about 23K miles on the clock). I have tried the "Super" grades (93 and 94) with the biggest difference felt in my wallet, so I stopped using them. I do run a good quality fuel system cleaner through the car several times a year (I try to do it before each oil change - 5K miles for me). The car is equiped with knock sensors that allow the car to adjust to a wide range of octane. Like I said before, I have not noticed enough performance degradation to justify high octane fuel cost. For turbos things might be different.

Hope this helps you. Please remember that the octane ratings in US are derived differently than in Europe and you should find a way do correlate the two.