If you look in the handbook, it will give you a whole range of viscosities from 0W30 right up to 20W50 based on the ambient temperature that the car will be driven in. Personally, I wouldn't fancy putting 20W50 in the car under any circumstances! I would suggest a fully-synthetic 0W30. At high mileages, I'd probably want to switch to a slightly thicker oil like 0W40 or 5W40. Go fully-synthetic either way to guard against sludging and to make sure the oil is up to the change interval enforced by the service indicator (ASSYST). Be warned though... 0W30 ain't cheap!
Cheers,
Ian.