I hate these oil threads, but here I am anyway, replying to one.
A local MB independent mechanic told me that one should never use 20W/50 in any engine. Of course this is just his opinion - he's not a lubrication engineer.
The business of initial lubrication at start-up was his concern - the first few secs. are the most critical and 20W/50 is simply too thick. Once again, his words; not mine.
I've taken several cars beyond 300,000 miles running 20W/50 year round. I live in a warm climate.
I think it all comes down to common sense and following the advise in your owners manual. Obviously a guy who lives in Canada is going to have different requirements than a guy in Phoenix.