I have not been on this site for a long time and I am amazed that guys are still targeting the valve stem seals and exhaust guides as the main cause of oil leaks.
There is good information already on this forum but I will hit the basics.
If you have a smoking BMW V8, burning oil the VERY FIRST thing to do is to pull the hose off the front of your intake manifold, right where the throttle body attaches. There are two hoses, one from each valve cover, they merge and plug into the front of your intake manifold. These are the engine vents or Posistive Crankcase ventilation.
BMW uses a different name but this is what they are.
They are supposed to pull vacuum against your sealed, air tight engine.
When you pull this hose off your intake, look inside. If you see oil you have a ventilation issue. These should be nearly dry. Some Oil vapor travels through the vent but no oil should be in there. Use a pen light and look inside the manifold where this hose hooks up.
If you see oil you have found your oil consumption issue.
To fix this issue you have to seal the engine up. There are many potential leak spots on these engines. If oil leaks out of this engine, air will leak in. If air leaks into the manifold, vacuum will pull oil into the intake manifold and you will burn oil.
This type oil burn issue will burn more hot than cold.
Actual valve stem failure will burn most on cold start and clean up after it runs a short time.
Exactly the opposite of what many BMW V8 drivers report but still they blame valve stem seals.

I will share that doing a valve stem replacment will fix this issue also because to replace valve stem seals you remove all the guilty leaking gaskets and O'rings and replace them as part of the valve stem seal replacement job.
Valve cover gaskets and all the O rings that help seal the engine will be brand new so the air leaks will be plugged and your oil consumption issue will go away.

I read that some have used Oil Stop leaks to fix this issue. It may help but I warn you very seriously that oil stop leaks do a lot more than plug oil leaks and so many parts in an engine need to be lubricated constantly you should not risk cutting off a small oil supply with stop leak.

Some of the components that can be the source of your air leak.
Valve cover gaskets
Dip stick O Ring
Vanos O Rings
The sensors that plug into the valve cover have o rings
the front covers on both heads have gaskets, if they are leaking, (mine were) they can pull air and leak oil.
The Vacuum pump on the front of the pass side head can leak oil and air.
The stupid O rings on your oil cooler hoses, I say stupid because the $6 orings on the oil cooler hoses can take 15 hours or more to replace. This alone is enough to hate the engineer that designed it. If these are leaking oil out, rest assured the Crankcase ventilation is pulling air into the engine. If air goes in, oil will be in the intake manifold.

Oil in the intake can't get there from the valve stem seals.

There are more potential leak spots but these are the main culprits.

If your engine has 250,000 miles it may need valve guides and valve stem seals.
At 70-140,000 miles. No way in my opinion.

I have repaired 3 of these V8 X5's and all three stopped using oil.
none got new vavle stem seals and all use 5-40W oil or 0-40W oil.
My 2008 X5 at 108,000 miles uses about 1/4 quart of oil in 5,000 miles and no longer has any oil leaks at all.

One side benefit to fixing all the air leaks is it corrects the vacuum signal to your computer which affects the way your tranny shifts. With all leaks fixed my tranny now shifts just like it did when the car was new. Incredibly smooth shifts. I had not realized how it was not shifting well prior to these repairs.
Gas Mileage is darn good. 20-22 on Hwy trips at 75-85 MPH.

I will finish with, I did not make any of this stuff up. I am Gold Certified as an Engine Machinist, Certified for Engine Repair and Engine performance and have 30 years in the business at the manufacturer level. I am not strong on repairs outside the engine but Internal engine parts I know pretty well.

I wrote this book again as I am pretty concerned when I see someone on here state they have Oil Consumption and exhaust blue smoke and 15 guys quickly tell the owner to replace his valve guides or valve stem seals. This is based on phooey not facts.
As I stated, if properly done replacing valve stem seals will include replacing all the gaskets and O'rings I mention so of course it will fix the issue but you are also paying to replace valve stem seals that don't need replacing.
If the engine is at 200,000 plus miles I would agree it might need valve stem seals but if you have taken good care of the engine with frequent oil changes the valve stem seals and valve guides should last longer than 200,000.

Start with the basics. Look in the crankcase ventilation tube at the intake manifold.
It will tell you what you need to know.

Then look for the sources of your oil leaks. If Oil can leak out air can leak in.
Air leaking in is what puts the oil in your intake manifold. It does not belong in there.
My son's 2005 X5 4.4L got new gaskets and O'rings at 145,000 miles 5 years ago.
It was burning oil like crazy. Now at nearly 200,000 miles it does not use any oil and does not leak any oil.
We fixed a 4.6l for a friend of his back then also. Same result.
Fixed my wifes 4.8L two years ago.
I hope this helps some of you. It is not magic, just engine basics.
Heavy oil smoke on start up after the car has been parked at least an hour = Valve Stem Seals
Oil in your intake manifold = Air leaks and external oil leaks.