I just started putting premium gas (91 AON) back into my 9-3SS. Honestly, I can't really tell the difference, between mid-grade 89 AON and 91 AON gas, in regards to performance. But at this particular chain of gas station, I am getting the premium gas for the same price as the mid-grade gas. It's not chevron, but I will see how it runs over time. Currently, the car is running quite the same as when it was on the 89 AON gas.