I pictured the pistons in the driver's side also. Attached is a one of them, all looked quite similar. This definitely looks more like typical carbon buildup so I guess Factory6speed is correct. I believe it was the same thing in the passenger side but somehow it was much more severe. I might just have to open up the engine a bit more to see how it looks like in there.

Attached Images

 

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09/2003 X5 3.0i
02/2003 X5 4.6is (non-runner)