With my old E53 X5 (M54B30) I found that it started to become unreliable and hard to maintain at about 20 years old (and only 140k miles). I was very religious with preventative maintenance but it started having weird problems that were difficult to diagnose.
The fuel trims were always off, it developed an unstable idle, and I was getting occasional codes indicating that there were vacuum leaks. I found due to the large number of parts that it was difficult to identify where the leak was. There are just way more parts in the intake and vacuum system than there needed to be. I took everything apart so many times and replaced so many things that I remember it like the back of my hand. I had a Foxwell NT510 scanner but the BMW-specific codes didn't help very much on this.
I threw the parts cannon at it but it was a mixed bag. I owned that car for about 8 years and really liked it - still wish I hadn't sold it (it was one of the rare stick shift versions) but if I was going to keep it I would have needed a third car so that I could keep it in the garage for long periods of time.
Regarding your original post, what is "standard maintenance" on a BMW? For a normal car I could at least make an educated guess at what that means.
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