My 560SL (145k) shows the same manual recommendations as mentioned above; 10W-30 for temps to 50F only, 20W-50 for temps always above 32F. Somehow at the last oil change a couple of weeks ago, 10W-30 was put in, and I since noted a light bit of blue smoke on starting off from rest after idling, more with prolonged idling and when hot, as well as consumption of a quart per 300-400 miles. No later than this weekend, I'll get the light stuff out of there and 20W-50 put in; I expect to see it improve.
I can think of no reason not to follow the manual on this!
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)