Thanks for the reply Paul. The only reason I asked was that I got told (by a non expert) that the high octane fuel has the ability to clean the fuel system, plus more MPG. I don't claim to be an expert in auto maintenance and thats the reason I asked here on this forumHi there Gael, I would think the only thing which would get hurt is your wallet.
Its only really highly-tuned cars (with turbos especially), that benefit from higher octane levels.
Its better to use a good, branded fuel, than supermarket petrol, as it contains various additives to keep the engine clear of debris. Although, occasionally using supermarket fuel won't do any harm.
Hope that helps - its quite a subjective topic - am a bit surprised that nobody's jumped in before me!