Southern California Gas and maybe the San Francisco area use a mandated additive in the gas for about half the year to help reduce emissions. In the San Fernando valley most of the people I knew were of the opinion, shared pretty widely throughout the area, that the oxygenated gasoline did not perform as well as gasoline without the additives. I've run my Altima on 87 since I bought it as that is what the manual says is required. 91 is an option for 'maximum performance', not really important in an Altima. I will use 93 in northern California while traveling through the mountains because the engine will ping on 87 accelerating up some of the passes. Here in the Seattle area the difference between 87 and 93 is about 10 to 15 cents, for the next grade up the price jumps about 20 cents. Someone will correct those numbers as I don't pay that much attention to them. We only have two refineries up here, BP and ARCO so virtually all the gas sold in this area comes from one of those two refineries regardless of where you buy it.

 

You buy fuel based on the Research Octane Number, not on the Anti-knock Index number!!!!! The pumps show the research octane number not the anti-knock index number. So the manual is saying 91 octane. I agree it should run on 89 but if there is a knock sensor problem, or very warm temps, etc. it should be checked out. But, they will ask are you running 91 octane and when you say no I am running 89, they will just say to run 91

 

I run 91 and I still hear what I think is pinging until the drivetrain warms up.. After a few hard accelerations, i no longer hear the ping noise.