what type of spark plug do you want to run? copper, platinum or iridium? I would also run out the gas that you have and see what happens when you fill up with premium. If you can actually hear the knock, that is obviously not good. If when you fill up with premium you are not knocking(which you may or may not be if you can't scan for it) you should not drop plug heat ranges. You only drop heat ranges if you have to, as doing so if not knocking will cause the car to run less efficiently and to get poor gas mileage.
