You should have an oil temperature guage too. If the oil temperature and the pressure are within the normal range, which means temperature below 120 C, and pressure pegged at 3 bar, you are fine. One of the adverse effects of a higher viscosity oil is lower flow. Lower flow will increase tempertures of the bearing materials, and the added resistance will increase the temperature of the oil.
So, it is really better for your bearings and the rest to have the lowest viscosity oil that will meet your car's oil pressure and temperature operating norms. I have used Mobil 1 15W-50 for years, but now I use 0W-40 Mobil 1 in my 1986 190E 2.3-16 with 206,000 miles. In any case the engine is like new on the inside. I just had the head off to have it rebuilt and the cylinders were like new.
The local temperatures here range from minus 10 C to around 35 to 38 C in the worst days of summer. I use the stuff year round, and have no problems with oil temperature or pressure. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)