Most engines nowadays have knock sensors. It'll retard the timing if pre-ignition is detected. I have used regular Costco gas for years in my 280SL and the timing is set to factory specs. There is no pre-ignition. That car has no knock sensor. I suspect that the fuel dispensed is the same for regular and premium. Check your gas milage: If you use premium you should get about 2 more MPG. It's supposed to have more energy content.