Taken from ford microfiche,

20deg C to 30deg C SAE 10W-30
Over -20C SAE 10W-40/10W-50
Over -15C SAE 15W-40/15W-50
0ver -10C SAE 20W-40/20W-50
below -20C to 0C SAE 5W-30

I do have the tolerances listed to run with 10W-30, but i doubt anyone will be running them unless on a fresh build. So for summer cars 20W-50 looks good.

Rob,