Oil viscosity - why the 20W-50 BS?

I put together my 360 with bearing clearances on the loose side and a standard-volume oil pump so I'm planning on breaking it in with Shell Rotella 15W-40 then switching to a synthetic 10W-30 or -40. If your engine has good oil pressure at idle when hot (i.e. just getting off the freeway and idling at a stoplight) with 10W-30 or similar oil there's no point in using thicker stuff; all it will do is rob power and gas mileage and make your engine more susceptible to wear during cold start-ups because it takes longer for the oil to circulate.

You should run the lightest-weight oil you can that still gives good oil pressure at hot idle; that gives you the most efficiency.