So I just made the leap and now have a 2013 X5 3.5i. My 4.4 e53 was as you mentioned maintainable with the trans and timing guide rails being the wildcards. That said the n55 engine appears to be maintainable though oil leaks are very common as are issues with the crank case ventilation handled by valves molded into the valvle cover. (The other common issues, gaskets on the oil filter housing on my rig have been done.)

My valve cover is leaking as a reward for new ownership lol, so I am replacomg the gasket and cover. Hopefully jumping on this stuff can save me trouble later.

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2013 X5 xDrive3.5i Premium
2002 530I Sport Package
2001 X5 4.4i
2001 M3