i think it logicly has to be the ECU needing to re-learn. which isnt a big deal, ither run a few tanks of 98 or disconect the battery and try again wich sucks.
but this raises another question, is the 98 realy 98, how would the average person know as they pumped it in, its all in the mind that your car is going better and as proven by kerry its not, at least not on the first tank.
as for me im still running PULP and i cant realy see the point in paying more for the product yet as it only drives from a to b and hardly hits boost.
another experiance i had a few weeks ago was i accedentaly put normal unleaded fuel in (it had been a long day) and i didnt notice one bit of difference. not saying this is a good idea just i cant tell what fuel im running.