I researched this for you a little and as far as Mercedes Benz is concerned, they recommend everything from 0W to 50W for various temperature ranges. 10W30 and 10W40 is shown in the same temperature range with an arrow pointing up after 30C.
GM strongly recommends against using 10W-40 due to the excessive amounts of polymer additives.
I would recommend using 10W-30 in summer and 5W-30 in winter. Unless you drive under the following conditons:
Extended speeds in excess of 100 MPH, in temperatures above 100F and/or 150,000 miles on the odometer. You will have to increase the tire pressure by about 8 psi in high speed conditons as well.
94 E320 58000 Miles