These engines were originally designed to be run on leaded fuel, which is no longer widely available. Tetraethyl lead was added to 4 Star petrol to provide the necessary octane rating (anti-knock quality) and to act as a lubricating barrier between the exhaust valve and seat in cars with soft valve seats. These engines weren't actually designed to run on a high-octane fuel - but can run fine on modern fuels, although you may need to adjust the carburetter. Do you have any history with the car? It's may have already had hardened valve seats fitted. If it hasn't, one option is to use a lead additive when you fill up.