Just an anecdote for you regarding guides on the straight sixes. I have an M52 in the garage right now pulled out of a car to swap into one that a buddy overheated and blew the motor on. I pulled the oil pan to do the gasket (because why not while it's on an engine stand) and low and behold; the chain guides had disintegrated and pieces were all over the pan...

It's still on the engine stand waiting for me to find time to do the guides. Engine ran fine BTW. No codes. Not even very noisy really.

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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)