I would stick with the 0w-40 in the winter months and use a thicker oil in the summer. If summertime temps regularly reach above 30C, you may want to use a thicker oil during that time.
Although, a a 10-40 or 15-40 will not be significantly thicker at high temps, they are all still pretty much a 40 weight oil when they get hot. And the Mercedes lubrication table I looked at lists any -40 weight oil as approved for use in high temps. Do you know how the oil pressure on the engine is behaving, that is the true test on what oil you should use.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record