Sounds like you did not start with the right amount of oil. My 2003 x5 3.0i owners manual says 8 quarts of oil. Mine is leaking from the oil filter housing gasket to. My brother changed the OSV pipes and did not help the leak. You are not seeing oil leaking because it has a catch plate. Under the car is a second plastic flip down. Inside is the oil plug, this is where the oil is draining to fromthe oil filter housing leak. I put paper towels to catch the oil for now until I fix the problem. Make sure you add oil if at all low and only run Synthetic oil. I run highmileage Mobil 1, 5W30. I change the oil every 5k miles and add about 1 quart between changes.
I have put 40,000 miles on mine with this issue. Keep the car but make sure you keep the oil level at the top mark of the dip stick.