I once took 6 quarts of Mobil 1 to a Mercedes dealer to use during a service. The *factory trained technician* came out and told me I didn't bring him enough oil, that his *book* said over 7 quarts were required. It turned out he didn't know the difference between an M103 and an M104 engine!
This isn't the only time I've had MB dealers screw up.
I don't trust anybody although I have had better luck with BMW technicians than with MB technicians. At least some of them treat you like a car enthusiast rather than a naive lawyer/doctor/bond salesman.
What I really hate are shops that do everything with an impact wrench.