Just bought a new '03 S60 - base model. The manual suggests 91 octane. Is this the right thing to do? I've often heard conflicting messages about whether or not there really is a difference between 87 / 89 / 91. Thoughts?

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

I forgot to mention about the test: accelerate full and 1/2 throttle in 3rd (MT) gear from 1000 rpm.

Note for a turbo engine, having a lower compression ratio (about 8.5 against 10.5 for the NA engine) you have to do the test from about 2000-2500 rpm, where the turbo kicks in.

Pick the lowest grade that makes the car run smooth and go with that. If the car runs well on lower octane, why buy expensive gas? Nobody here can prove beyond a shadow of a doubt whether high octane really does what oil companies claim.

My car should run on 91, but I run 85 with no problems. I ran 91 for a long time and found no noticable difference.

Rick in Denver

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