I have a 2002 X5 3.0 which leaked some oil. A common issue with them is the seals for the front drive shaft will wear and those seals have to be replaced. It is a fair amount of work - axles have to be removed. In fact when doing it I would replace the front axles as well which gets you new CV joints and boots (often cheaper to buy new axles than just replacing the joints and boots). I got new axles (which have lasted just fine) from Advanced Auto Parts for around $99 each. Stealers will sell for a lot more of course. To be clear, the issue is likely the shaft seals leaking, NOT the CV boots or axles themselves, but since those have to come off to get to the seals, may as well replace them too. All that said, a close inspection could reveal a leak from somewhere else (oil pan gasket for example). If you have to replace that, again it is a big job as the front drive goes through the oil pan and the above described work still has to happen. My leaks all went away with the replacement of those seals and oil pan gasket.
Article here from Pelican Parts on what is involved.
Get a good aftermarket extended warranty that covers leaks and seals. I bought one and already made back $1k in repair costs, thank you oil pan gasket, oil filter housing gasket, and CV shaft gaskets. Wish the RCB unit was covered under the electrical components...
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