I can use either of the above in my new octavia.

The 98 RON is about 6p dearer than the 95 RON where I live (91p/litre) and am wondering whether I can expect a better than 6.5% improvement in fuel efficiency in using the more expensive fuel?

Also are there any other benefits in using the 98 RON fuel over the 95 RON fuel?

I believe Skoda advise using 98 ron fuel in the FSi engine as it aids the direct injection combustion process.

When I took delivery of the car the dealer said "you can fill it up with the cheaper stuff" so I assumed this was the norm.

What is the effect of aiding the direct injection combustion process?

You can run it on 95, but as stated in the cap 98 is prefered.

Im guessing its due to the higher octane rating meaning the fuel burns better and maybe quicker which is essential for FSi to work.

Hi

Evo a couple of months ago tested various feuls through the FSi engine in a golf GTI and found decent but variable gains using premium fuels.

Chris

FSI engines will run as a normal engine when you use normal fuel (as explained to me in far more detail by a top level tech from VW UK HQ at a VW Press launch).

They only work in true FSI mode when you use something like Optimax in them.

They only work in true FSI mode when you use something like Optimax in them.

Good press quote that. However its rubbish..lol. They are essentially direct injection petrol engines, to be a true FSi and run in stratisifed mode you need a NOx cat and another lambda probe in the tailpipe. As yet i've not seen it.

But they do produce good torque and MPG so its not all bad.

You may be interested in this thread which covers the topic fairly conclusively,

http://www.briskoda.net/forums/new-octavia/super-unleaded-97-ron-fuel-octavia-vrs-mark-ii/49914/

Bear in mind the difference between the FSI and the FSI Turbo. The former probably copes quite happily with 95 RON but the Turbo will perform at its best with 97/98 RON.