my uncle used to drive a ford taurus SHO. and it was recomended to run only 93 in it. BUT he found that running 87 or 89 octane gas made it run better. He assumed this was because most people dont buy 93 so it sits in the tank for a while and get old. He used to just put in some octane booster in the 87 to around 95 and it purrrrd like a kittin. and the car was fast as hell.
higher octane just helps if you have different timing