[Gerry]

Whether there is an advantage or difference in how the engine performs depends upon how you use the car. If you're light on the throttle and the engine doesn't normally see high loading, then there won't be much difference. If you take advantage of the engine's power, then there will be a difference. It comes down to how the knock sensor is managing the ignition timing curve. The blatant evidence would be born out of the datastream for the ignition advance. Lower ignition advance means less power and lower fuel economy. Higher advance gives you the opposite. It's a tough call and you can't apply a universal answer to every situation. And it's hard to make the call with just one blended (some regular and some premium) tank of gas. I'd say try each for a month. This will give you higher resolution in your measurements and you can make a decision based upon the more abundant information.