servicing the car at authorized service center really that important for expensive cars??..

IMO, authorized service center does not mean they are good/specialized.... sometimes even worse as they do employ trainee and interns as mechanics....

check with the service center what kind of engine oil they put into ur engine... i suspect bad workmanship as well... engine oil level decreasing means something is wrong, and they still can say its normal... even with or without visible leak, it is not suppose to happen...