difference i feel is the boost cuz i can crank it to 25psi. i had 91 octane in yesterday, same idle, i even checked it with my techtom. you sound like my buddy who says "oooh ,my car feels soooo much smoother and faster cuz i changed my oil"!! buahahahahahhahhahah but anywho, everyone's z is different, maybe you z does run better at idle.