Originally Posted by Schuey
BMW recommend at least 91 octane as a bare minimum.
That depends on your model. The E36 was told to use 89 octane or better. As such most just used 91 as the standard. BMW states that using anything less then 89 can cause problems with your O2 sensors and your Catalytic Converter. The extend of this im not certain, but it is a fact that is listed in my owners manual.
I dont believe anyone mentioned the Idle Control Valve. Mine was stalling when i didnt give it gas and when we replaced the ICV it solved the problem. A dirty or worn out ICV may be the cause of the issue.