I don't think there is any definitive answer for valve stem wear. There seems to be speculations that the cause is from extended oil changes etc.. which has yet to be proven. I beg to differ on that note. BMW just simply made a bad design of the N62 engine.
13 740LI W/Msport package. Executive Package
DRIVER ASSISTANCE PKG, HUD,
OYSTER; NAPPA LEATHER SEAT TRIM,
Lane departure, blind spot detection,
Side & Top View Cameras
Aftermarket Rear entertainment