Interesting comment on the 20-50 in the 1991 300SE. When I bought my 1991 300SE in West Texas 20 years ago, the dealer was recommending Kendall 20-50.
The owners manual recommended all the usual oils, but the 20-50 was the heaviest oil recommended, and not suggested to be used below 32 degrees.
It does get hot out there, and folks drive really fast, so I can see the logic in the 20W-50 recommendation.

But, I'm a subscriber to the approach that initial cold start is where the vast majority of engine wear occurs, so I went with 10-40, and then switched to the Valvoline Max Life 10-40 when it came out about 10 years ago. 220k miles and still going strong with very minimal oil usage between 4-5k mile changes, and very infrequent seal/gasket leaks.
I like the Max Life because the adds say that it is formulated with additional detergents, a bit more seal softener, and is taken from the high end of the viscosity range. I don't know if all this is true, but if it is, it sounds like all the things we should be doing in an old engine.

I've run all kinds of oils in all kinds of engines for 50 years, and never had any kind of oil-related failure or wear issue in any one of them.
Yeah, technology changes, and 1991 300SE's are running hardened flat-tappet cams while modern motors are roller lifter. But the same approach seems to still apply - use a good brand oil in the viscosity range recommended, and change it pretty often, and worry about something else.
Works for me.

DG