If you read the info that IPD publishes on their web page concerning their ECU upgrade, you'll find that the engine management for the the S60 is very advanced and one of the best out there. The engine can and will adjust to lower octane then it recommends. This will result in decreased performance for the turbos but mostly no noticeable change in non-turbo engines. You can run regular 87 octane in your car and not damage it at all, but you will probably notice a DECREASE in fuel efficiency because the car will be burning more fuel due to a lower burn point (catches fire easier).

This of course assumes that the gasoline you're using is actually the octane you pump - and I hate to sound like some sort of conspiracy theorist but I believe that at least in the state of Georgia the gas I'm getting typically is not what I paid for.