Dave ANY normal unleaded fuel is fine.

You WONT see any performance gain buy using optimax/super unleaded because the ST170 has no way of detecting the different type of fuel and making the nessasary timing adjustments, so its a waste of money buying optimax/super unleaded for a std car

Now if you have a bluefin fitted there is a bluefin setting for using optimax/ super unleaded that DOES make a difference to the car because it reprograms the ECU and adjusts the timing for the high ron rating optimax/super unleaded has

I use the cheapest unleaded in my std ST170, which at the moment happens to be shell and sainsbury in my area

HTH