@Ard:

As I stated in my first note (#18) on this thread, the engine got a P0306 code (for cylinder #6 misfire) and ran rough and stall. I replaced the ignition coil and spark plug and the engine still runs rough but less stall and continue to have code P0306. So I moved on to replace #6 fuel injector. The engine runs slightly better (I have taken it on to local streets) but still runs rough on acceleration, and still has the code P0306 with the check engine light ON.

As M5James mentioned in post #14 with the last sentence that "they (meaning injectors) NEED to be coded so the car will run correctly with them." It is a reasonable guess that a genuine BMW fuel injector may not require the re-coding.

Experts like you and M5James may be able to tell if non-OEM fuel injectors would work on this X5 3.0si and if they need re-coding into the computer and how. I really have no clue.

I read through this thread and some have suggested that vacuum leak and lean condition could also cause misfire codes. But how do I identify them?

Yesterday I remove the newly installed #2 spark plug after about 15 miles and it has black coat on the tip. Does that mean "running lean?"