When I first got the car I ran it on 91 octane, but the person who owned it before me drove it light so over time I freed it up quite a bit. The amount of crap that would come out of the oil was incredible, that engine was dirty as. Now after a few flushes its clean and I started using 98 octane some time back and did notice a gain in power and mileage, but this could also be due to freeing up the engine from its 'granny' state. I could try 91 octane again for a couple of tanks and see how that goes, but if there is any loss in power or miles I'll go back to the 98. I was referring to gutless as in the cars performance between the two different fuels, not comparing to other cars in general. Its not a race car but I'm a bit of a lead foot anyways