I normally use superplus unleaded in my GT6 (97 octane), and put Optimax (98 octane) in when I can find it (like this morning). However, I have
found the power difference to be marginal at best. I do prefer it though for it's 'cleaning' properties. I would say though if your engine is
'tuned' to use standard unleaded you will notice a marked difference, but you will need to change your timing to get optimum performance.
When I filled up this morning and pulled out of the petrol station straight into 60mph traffic - the car pulled cleanly and powerfully, and that
would've been before the optimax had reached the carbs.
When I tried it in my eurobox (now sold) it made no difference at all. This had an engine tuned to use 97 octane though.
NiG
http://www.niginthecar.net