Tricky question. I remember reading a long time past (either Autocar or What Car) that the premium petrols of 98 or 99 octane could be better for your engine, particularly if it had ideas about being “sporty”. So when I’ve had a petrol car, I’ve always used the premium. Had a quick Google on subject and there doesn’t seem to be any decent science on it. Must say, however, that when I was forced to use standard octane in car (Cooper S) because the (Tesco) premium wasn’t available, I definitely noticed that the engine ran much more smoothly when it went back to premium.