spent a week cruising the hinterland behind coffs harbour last year while the old girl did some business there. I had the luxury of a BP, Shell, and a caltex within 5 minutes of where i was staying so i tried them all. The caltex returned the lowest knock reading of them all. Only talking a couple of points and, as has already been pointed out it could just be different batches causing the slight variance But i wouldnt worry about using any of those 3 in a car tuned to 98 fuel.