Lude Generation

On my 1989 Prelude 2.0 (B20A7), it has a 'this car has been converted to unleaded' sticker inside the fuel filler flap. It also says 'use only 97RON fuel'.

Am I right in thinking that the 'unleaded conversion' would have been simply retarding the timing? If so, can I de-advance it to get full performance with Super Unleaded? And does anyone know what this should be?

If it's a UK car it's probably been converted to use unleaded from 4 star petrol.

I used to fill my prelude with shell super unleaded but didnt spot any difference... Filling with a normal one and happy