You're wasting your money buying 98 unless:
-you have an ultra high compression engine.-pressure built up in the cylinder will cause ignition before the spark plug fires.
-you have an old beat up motor with carbon deposits that may ignite the air/fuel mixture prematurely thereby causing predetonation, pinging etc.
Remember, the higher the octane the more resistant the fuel is to ignition.
I just love Shell, where the guy tries to convince me that their 99+ will make my Punto run faster.
Used to be that the premium gasolines had more cleaners and junk that were supposed to make your engine run better and last longer but now I don't see that marketed any more. I suspect that due to fuel content legislation there is no difference between additives to lower and higher octane fuels.