How interesting is that picture!!!!! My car too was missing the lip in front on the wheels: I thought someone removed it at some point and forgot to put it back... Maybe it was never there to begin with... Just like in your case there were the 3 metal inserts, but nothing attached to it. It's part #3 here: RealOEM.com * BMW E70 X5 3.5d Wheel arch trim, front.

If you look very carfull in these picture you'll see them: Review: 2009 BMW X5 xDrive35d Photo Gallery - Autoblog

Now to get back on topic: what oil coolers are you referring to? I'm not aware of any oil cooler in that area... On the driver side there's the auxiliary water cooler and the passenger side should be empty.

The only oil cooler I know of is the power steering one; the transmission oil is temperature controlled through a oil-water heat exchangers. All of these are in the center so if they were to leak I would expect traces of oil elsewhere

If it does indeed come from the intercooler (that too is in the center) I would question what and why oil is in there to begin with...

If I were you I would actually remove the bottom plastic and see where is it coming from.

Another possibility is that is just residual cosmoline (the protective oily stuff they put on the engine during transport) that was not properly washed off.