I don't have a manual for 3L sixes or 190 4 cylinders, but doesn't 20-50 seem a bit heavy?
My later model GM and Fords call for 10-30 and 5-30 ...
I use a much heavier oil in my 350SDL ... lots of differences in tolerances and needs, though.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg