Originally Posted by dlh1999
IMO, you'll probably want to switch to a 91 octane map pretty soon. You're basically making your current map lean out the car a little because it expects 93 octane but you're only giving it 91 (which, I hear, isn't great quality in some parts of California, Nevada and Arizona). It'd be pretty safe going the other way (ie. 93 octane with a 91 octane map) because rich is better than lean, but probably not the best idea vise-versa.
octane doesn't have much to do with the AFR. the AFR won't change (much) with different gas. what does change is if the car knocks or not, at a certain RPM it might need more fuel to prevent knock, even though it technically isn't lean, or at least any more lean then before.