My siste visited Lisbon and she loved it.

The only way to diagnose oil leak is to remove the bottom aluminum shield and leave it out for a while while you drive it daily and check for new oil leak (of course, clean old oil leak first). If you google "xoutpost E53 cn90 rear diff", you will see the DIY I posted. If you leave the bottom shield out, you need to re-install at least the 2 bolts holding the swaybar in place.

I don't know about diesel model, but for gasoline model (mine is a 2006 X5 M54 engine), common sources of leaks at 100K miles are:

1. Oil Filter Housing Gasket. Use BMW dealer gasket only.

2. Valve Cover Gasket.

3. PS Hoses where they are attached to the PS Pump.

3. The Oil Pan Gasket does not leak until 190K-200K.
My 1998 528i M52 oil pan gasket is still bone-dry at 170K miles.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT