From my understandning, you should ONLY use super unleaded or better (97+ RON) (Optimax is slightly better than normal super I think, 98.6 RON?) the reason being I think is because turbo cars run slightly lower compression so require a fuel that burns at a higher temperature to avoid pinking.

The japs are lucky and have 100 RON fuel available over there, thats why if you buy a fresh imported car (scooby etc...) you need unplug it's brain overnight so that when you put it back in the following morning and fill up with your 97RON fuel it will reprogram itself for use on the fuel (kind of like an automatic "Live Map")

Im not 100% sure of all the exact details, but thats a rough idea as to why.