I didn't realise you could either, at least until my brother told me. A few years back when he ran a Caterham 7 with a Ford x-flow, it normally drank unleaded with an LRP additive, while really wnating 4 star. One day out the blue he passed a garage serving 4 star leaded and needing a tank full pulled in and started filling. Luckilly for him he noticed the (worse than mentioned above so far as I can recall) price and stopped with what he had put in.
Personally, never seen it. BP 102 ron unleaded still my most expensive fill!