well over time if the seals go bad.. or if you use bad fuel.. the seal leaks and oil started going into turbo..
Beside this most of turbocharged engines consume a little bit of oil and you need to fill up to required level between the oil changes. So you must check oil levels after every 4 to 5000 miles. Initially after taking delivery its always better to check every 1000 miles so that it gives you an idea that how much it consumes . Mostly the levels remain stand still at the same level. Its normal to consume a little bit of oil but if its drinking a lot you need to get it checked.
Here people mostly use Synthetic oil for the turbos as far as I have always seen.