Okay, Bob, here are my qualifications: I am an ASE certified mechanic with over 35 years of experience in performance (including OBD-II) applications. I have read my owner's manual, and I see no conflict with the Volvo engineers in using 89-octane fuel. My manual states that for "optimal" performance, 91 AKI fuel should be used, with a minimum of 87. With the current cool weather, low humidity, and light performance demands of my wife's around-town driving, I would be foolish to pay for 93 octane (the premium AKI available here). Those at high altitude might well find that 87 octane works perfectly well. In extreme heat or when I am expecting a full load I might well go for the premium, but I don't see a need for it in most applications. Your point to read the manual is well taken, but it has been my experience that more motorists use fuel octane rated too high than the converse.