When I first got my car with the original alfa air filter I tried shell optimax. It did not seem to change the performance and after a short while I stopped using it. Some time after I fitted a K.N air filter which improved performance and induction sound but seemed to create a rough tick over. Last week the local petrol station had run out of normal unleaded and therefore filled the car with optimax. The rough tick over has disapeared and the performance has improved even more.