leaded fuel will build up on sensors (o2) and running a super high octane can result in very high EGT's and a loss of power if you don't tune for it
so your answer is a stock un-modded car 93 oct
stock modded (clamped) W/ high boost 97+ oct depending on the clamp
remember higher oct burns slower (and has a higher flash point
) so it will do almost the same thing as retarding your ignition if misused
