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This is a topic that has been discussed many times at various places.
I have a S60 NA too.
The most important item w.r.t. octane is that the engine will not ping (detonation/ pre-ignition/ multi flame fronts, etc). In particular when temperature is rising and humidity is rising, the engine is more sensitive to pinging. As soon as you hear a (slight) pinging you should switch to a higher octane rate. During extreme hot and humid wheather I would always recommend the highest rate, even for a non-turbo engine. The knock sensors of your S60 will detect pinging and the ECU will retard the ignition.
Personally I use EU RON 95 (which is a midgrade comparable with US 89, I believe)up to about 25 deg C (about 80 F). Above that I use EU RON 98 (comparible with US 91).
A simple test to verify if there is pinging (only for NA engines):
With a fully warm engine, accelerate full and 1/2 throttle in 3rd (MT) gear and listen carefully with the window open if you can hear pinging (metal-to-metal sound). Don't do this to offten because it will stress the engine, although it is for just about 2 seconds and can do no harm.
Regards,
Dutch
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