I was forced to drive my 84 300td for part of a winter due to someone crashing into my subaru. When it got below about 15F the engine had a hard time cranking over on delo 15w40. When I changed to rotella T6 the engine started up perfectly fine without being plugged in at -25F.

Unless you're operating your car in extreme cold, use what has been working. Except for cold starting performance I saw no difference between the two oils, didn't leak more or less, didn't run any differently, no difference in power, etc.

I'm no oil expert so I just go by what it says to use in the manual. Using a straight weight oil is a little bit outdated, but whatever multi-grade oil they suggest to use is probably fine. Someone was paid a lot of money to engineer the om617, take their advise.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Correct to consider the zinc content. In the classic car world (gas), most engines are flat-tappet lifters and some have problems with excessive wear. But, seems most of those are people running much higher than factory spring loads with much higher-lift cams. There are oils marketed for "high mileage engines" that have more zinc. Some classic car owners use "CJ" rated diesel oils, both for more zinc and more detergents. My guess is that with the normal M-B cam and springs (are there even alternatives?), you won't have a problem from using any diesel oil. U.S. OHC engines I have had (1982 & 1996) had "followers" on the camshaft similar to my 1980's 300D's, albeit hydraulic. No problems in those engines running regular "gas" oil, unless the upper end starved for oil.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans