20W50 is good for a warm-hot "Mediterranean" climate.Definitely do not use 20w50 if you park your car outside. Once you park your car outside the engine is really cold thus impeding cold startup oil flow. You dont mention if you have a 300E but MB used 20W50 since day 1 even in cold climates.
Another way to determine what viscosity is best for you is monitoring your oil pressure gauge. If you see it goes down to 1 after a couple of minutes then use a heavier weight. If you mostly take the car on freeways use 20W50 but if it is a short trip car, use less viscosity. The m103 has an iron block thus it is hotter than your standars Al engine, so dont use a 5w20 or anything silly like that.