There is a video on Youtube of a complete renovation of the engine of a mid-2000s 325i. Look for M539 restorations. He replaced lots of parts, particularly on the intake side. That vehicle was also fouling the plugs and his conclusion was that a blockage of the oil return to the dip stick tube caused it. Not sure if the CCV design is the same on a 3.0L e53 but possibly some similar issue?

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.