From my own experience and from others on this forum the 70 to 90k miles range seems to be when the X5 gets all sort of issues like water pump, valves cover gasket, CV joints, steering pump, etc... all fixable but pretty costly if you do not have an extended warranty. Mine 4.4 passed 110k recently and has been trouble free for the last 10 months. I was lucky to have an extended warranty when all these problems occurred. My transmission does not show any sign of weakness so far.