should i use is 89 or 91? ? I heard most BMW owner's are using 91 but the former owner he said he was using 89 octane.

use 93 octane due to fact that cleaner running fuel, beter cubustion, beter gas milage, more miles to the galon. it may cost you a little more but its beter when it comes to all that carbon build up. I have a 89 mustang 5.0 so does my budy when we replaced our head gaskets on both coups, the heads that were using 89 octain his of cours was full of carbon build up especialy on the valves. on my coup 5.0 it was still nice and clean did not look bad at all becaus i use only high octain and that is 19 year old stang.