In the E39 forums, bimmerfest, bimmerforums etc., the general feeling is:

- For Dual Vanos M54 engine, when the Vanos seals go bad, the engine can stumbles etc.

- The V8 Vanos is less troublesome, even if the seals are bad, you don't have stumbling issues (I may be wrong). The bottom line with V8 is: leave it alone until you have to do things like Valve gasket, Valley Pan etc.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT