Are you sure your car specifies 98 OCTANE?
I think it says 98 RON (Europe spec). I believe this is the equivalent of 91 or 92 OCTANE.
BTW: if you do run higher Octane, then you should be able to run some higher boost. I am sure one of our many fine engineers on this board will have a better answer for you.
Have FUN!
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Nick Hromyak
'85 Carrera 7 & 9 Fuchs
Havin' Fun in Sacramento