Valve stem seals. If you let your truck idle for longer than 5-10 minutes, do you get a plume of smoke the next time or two you hit the accelerator? Do the plumes happen after leaving long stop lights, in stop and go traffic, and possibly even at drive thru windows?

If you answer yes to any of the above conditions... your N62 needs valve stem seals like 90% of the other N62 powered BMW's at your mileage. My 4.8is needs them as well. I'm guessing it was the reason the truck was traded in @ 90,000 miles because it is an expensive repair. I have 113,000 miles now and have been mitigating the need to add oil (I still add about a quart every 3000-4000 miles) by using a heavier rated (hot rating) oil and using a treatment of Lucas oil stabilizer. I rarely have smoke now and will eventually do the VSS, but this keeps me going for now without looking like I am driving a 70's Cadillac.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)