Take a look at your owners manual, it will list which oil viscosities are acceptable at which temperatures. Mine lists a 10W30 for temperatures below 50F.
I also had my mechanic change my oil and he would charge $80 for the oil change using Valvoline Synthetic 15W40. I didnt have any problems but I decidede to start changing my own oil and spending the labor on better oil. I now use Amsoil 10W40 Full Synthetic and the car runs and starts so much better.
02 C32 AMG
84 190E 2.3 5 Spd