What grade of petrol should I be using, 91 or 95?

You can check the recommended fuel for your vehicle in your Owners manual. Generally a vehicle can run on either 91 or 95 octane. The vehicle will perform better on 95. 91 may be cheaper at the pump to fill however, you may get more distance per tank (better efficiency) on 95. So, even though it may more to fill at the pump, but it could work out cheaper per km traveled.