When knock is detected timing is retarded, no? How could that not effect performance? Why would GM tell their customers to use premium if it isn't required? Wouldn't make sense marketing wise to increase the cost of ownership for no reason. Even my '01 owners manual states that using lower octane may decrease performance.Sure you can. It has a very effective knock sensor. I ran my '97 on 87 octane for years and never noticed any difference (performance or mileage wise) . Give it a try. You won't hurt a thing.