@OP everything you are describing sure sounds like a compression problem. Bad valve guides can sometimes not show up on a compression test.
I think number 5 makes most of the brake booster vacuum so something else to check.
There is no "normal" amount of fuel on the spark plug so thinking that the cylinder is getting too much fuel based on that is the kind of thinking that will lead you down the garden path.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)