My 4/4 Sport is the same year as yours and I have only ever run it on 95 octane unleaded as per the handbook and it runs perfectly. Being a humble Ford engine designed to run in the Focus 95 octane is kinder on the pocket! Somebody did suggest to me recently that it would be a good idea every few tanks to put in some 98 octane to clean it through but I haven't done it yet so don't know how it affects the performance.
