I'd like to use cheaper gas if I can, although here in CA the difference between 87 and 91 is less than elsewhere. Don't worry, we more than make up for it.
From my 2001 X5 Owner's Manual, page 30.
"Fuel specifications
The engine uses lead-free gasoline only.
Required fuel:
> [B]
Premium [/B]Unleaded Gasoline,
min. 91 AKI.
AKI = Anti Knock Index"
I've seen different Octane numbers before, but this AKI is new to me.
They did say "Premium" though.
The main concern I would have is if damage is occurring without me knowing it before it is too late. But for me, I'll keep using Premium, if only because I don't know better enough to use 87.
EDIT - quick google suggests the AKI is the same as USA Octane, so 91 AKI is true, Premium, mega-dollar, 91 Octane gas. I'm happy to be proven wrong.
this reference talks about how AKI fits with the more common ratings.
https://www.bmwmoa.org/news/438324/U...-RON-oh-my.htm