I would start by getting it on a lift, removing the reinforcement plate, and see if the source of the leak is obvious. If not, the next step is cleaning everything with de-greaser, then bolt the reinforcement plate back on and drive the car a few days. Chances are it's going to be quite a messy job, so be prepared for a huge clean-up of the shop floor. After driving a bit, then get it back on the lift and look again. An option is to add UV dye to the oil, and look for the leak with a UV light and yellow glasses.

And don't forget that when you're ready to button things up underneath, you need to replace and properly torque the bolts for the reinforcement plate. They are torque-to-yield "single use" bolts.

Given the fact that we are talking about an X5 here, don't be too upset if oil is leaking from more than one spot.

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2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp