Likely not MAF or O2 sensor, but more likely a vacuum leak as a lean mixture indicates unmetered air getting into the manifold. I would check the crankcase vent hoses at the rear of the intake manifold as they are a common failpoint on the V8's at higher mileage.

With the plastic cover removed look at the rear of the intake manifold for these hoses... #'s 4 and 5. If they feel like modeling clay, that's likely the problem...

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X5 pics

RIP 4.6is.....

2003 4.6is