I'll buy the explanation that back then the Mercedes engineers didn't think much of the 5w-30 mineral oils.
Of course, it begs the question that with today's SM mineral 5w-30, would the Mercedes engineers still hold to this recommendation? I've read in Mercedes forums where drivers for these old cars have used everything from 5w-30 to 20w-50, with no lubrication problems noticed. I'd say most are using some kind of Xw-40.
I've never had any problems with the 15w-40, even in the winter around here.
I had a wheel bearing replaced on the old BMW last month, and the indie mechanic changed the oil for me, putting in Mobil conventional 10w-30. It has an oil recommendation suggesting Xw-30 oils are for winter temperatures. There's only about 700 miles on it. I do notice it revving a little quicker than with the 15w-40 weight oil, but maybe that is my imagination, who knows? Otherwise, no noticeable difference.
My gut tells me that you could probably use Xw-30 conventional oils and as long as you aren't seeing any consumption problems or are taking it to a track, you probably wouldn't have any oil lubrication issues.
Give the 5w-30 conventional a shot and tell us how it goes, I say. There are those of us out there who would be interested in the results. But hey, I'm enjoying reading G-Man's experiments with the straight 30--that's so old school is sounds new and original!