BMW sixes with over 100,000 miles often develop an oil leak at the lower rear corner of the valve cover. I'd suspect that's the culprit. My wife has an X3 2.5 with nearly 130,000 miles on it and I will have the valve cover gasket replaced at the next service. Aside from that all I've done is replace the serpentine belt at 120,000 miles, brakes at 60,000 miles, and tires every 45,000 miles.

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1975 2002A
1995 318ti Club Sport
2004 X3 2.5
2009 328i
2007 Mazdaspeed3
1999 Wrangler Sahara
1996 Speed Triple