Just had a word wth my dealer regarding WR Sport kit for the RB5 regarding NUL and SUL,
the official word from Prodrive is for maximum performance us SUL but ita ok to use NUL on all the MY99 cars fitted with the kit. But NOT on pre MY98 cars at all! seems interesting..

The official line is that the MY97 onwards Prodrive conversions (smaller turbos) can be run on either Super 98RON or standard 95RON fuel. In all cases the car must be supplied with a tankfull of 98RON fuel.

On the pre MY97 cars with the larger turbos, the results seem to be better as the larger turbo allows for a bigger improvement, but at the expense of 98RON Super unleaded only.