Oil viscosity you decide to run all depends on how tight you run your bearing clearances. In many cases, 20w-50 isn't too thick to run on a rebuilt Duratech if your clearances are set a bit looser than stock. I think stock Contours run pretty tight from the factory, so I personally, only use 10w-40 at thickest in the summer in mine. But I wouldn't run 20w-50 on a motor that has .0005 to .0015 bearing clearance though. It might chance starving the bearings for the first 10 seconds of cold start. Maybe not significant time, but it is extra wear nevertheless
