Depending on which engine you have...the information from the owners manual can be different. As some have stated above, take that information...and the rest is up to you. Do take note of the meaning of
"recommended" and "required". Also know that elevation to sea-level, topographical terrain (hills or plains), and temperature can all have an affect on performance or the demand you need from the engine. And the type of driver you are can make a difference as well. If you have a gentle foot or (granny-toe)...your driving demands from the engine is going to be different than those that are lead-footed.
My e30/e32 (m20/m30 engines) only required fuel with an 87 AKI...higher octane fuels would be a waste because it didn't have knock sensors to adjust for different octane.
AKI is the calculation used in the USA to determine octane...RON & MON are used in other parts of the world...and have slightly different numbers due to the way octane rating is calculated.
(87 AKI is equivalent to
91 RON)
For my e34, e38 & e39...87 AKI, 89 AKI & 91 AKI is actually mentioned.
(my (e34) 525iT w/M50 VANOS engine recommended 91 AKI w/87 AKI as minimum requirement...my 740iL w/M62 non-VANOS engine recommends 89 AKI w/87 AKI as minimum requirement...my 540iT w/M62TU engine recommended 91 AKI w/87 AKI as minimum requirement)
The fuel specs for my 06 X5 4.4 w/N62 engine gives no other "recommendation" and says that it "requires" 91 AKI: