Read the article beginning on page 3 of this link:

http://stlouisncrs.org/news_files/St_Louis_NCRS_Chapter_July_2011.pdf

I don't know about more recent Merc engines, but my M103, like my '63 Corvette, Cosworth Vega, and '91 MR2 have sliding surfaces in the valvetrain, so I follow my own advice and use C-category oil in all.

Given low mileage, low rate of mileage accumulation, and low oil consumption on the catalyst cars, I'm not worried about aging the catalysts.

Duke