I think 20W-50 is a little heavy. I will probably cause no harm but your gas mileage will suffer a little. I personally think 5W-40 is best but that is hard to find in a synthetic. Mobil makes Delvac-1 in 5W-40 but markets it only as a diesel engine oil. It will work fine in our gas engines but you can only find it at truck stops or Mobil distributors. Allegedly Mobil is making a special 0W-40 that is available only at Mercedes dealers for big dollars.
I take the following approach. I the winter I use 5W-30 Mobil 1. It the summer I use 5 quarts of 5W-30 and 2.5 quarts of 20W-50 - all Mobil1 synthetic. YMMV. It'll end up around 10W-40.
1984 300D Euro