Your octane rating is based on a different manner. I believe it is a "Research Octane Number" or "Motor" octane. In USA, we use a combination of those 2 numbers to get an average of them, R + M divided by 2. Usually, the methods that other countries use is a higher number than ours. In other words, YOUR 98 octane may be the same as our 92.We hawe 98 octane and I add octane booster at evey fillup .If the problem was timeing related it would not follow the rpm from idle to wot whitout changeing .