loss of engine oil?? on 2000 BMW 323Ci

every 3500 km i have a loss of 500 ml engine oil.why?

Asked by for the 2000 BMW 323Ci
If you don't see any external oil leaks, then you could have worn valve guides or worn piston rings. There are other possibilities too, but can you see any smoke from the engine? If so, when does it occur?