Marbles in any motor isn't good no matter what fuel you use. Seriously, if you have a modern engine and there is a noise associated with using an octane rated gasoline 2 points below the required (by a manufacturer who might have some sort of an incentive to requiring a certain octane rating, ie Mobil1 oil required) then you have a problem associated with that particular motor above just the gasoline. Dirty gas, watered down gas, are just two examples (of a huge list) of actual problems but these are exceptions to a rule. Remember, I am not talking about modified engines or overhauled engines in engine bays, just your production run engines under a hood. Older carbureted engines with distributors, older motors dropped into modern cars and spliced in, or new motors not designed or completely installed with ALL the designed sensors installed correctly, all bets are off on those. I stand by 89 octane from a reputable station (remembering even they can get a bad load on occasion and no amount of octane in that gas will matter) for a car that a manufacturer requires 91 octane. Published specifications posted/designed for are why a specific octane rating is or may be a requirement for this or that auto engine. Engineers MUST build from a specification starting point. As a matter of my experiences and research to include articles published extensively (and no I have no numbers or copies to 'prove' this stated conclusion), modern engines with the level of sensors and computers equal to our 3.2L motor will not be harmed by using 89 octane if the 'required' octane rating is 91. Remember, they also say in these manuals that NO ADDITIVES are to be used yet you consistently see this person or that person dump this or that into their motors without hesitation. The bottom line IS the bottom line. If you want to spend the monies, or not, it is up to the one pulling out the money that ultimately makes his or her decision. You are not going to 'hurt' your motor by purchasing and using 89 octane. Heck, some even buy 93 octane, that is also the choice of the one pulling out the cash. I, in good faith, cannot tell someone 89 octane will hurt their motor if it doesn't. It is and always is a matter of choice based on the totality of the research I have done and I burn locally sold 89 octane. Economically speaking (and it is the ONLY consideration) it is just misinformation (IMHO) to be spending another $0.20 per gallon for 93 octane here, for our 3.2L motors. Peace out!