Oil viscosity - why the 20W-50 BS?

I'm using 15w-40 conventional diesel oil with Comp Cams break in lube to break-in the engine, then racing with 5w-20 Amsoil Dominator synthetic.

All the new vehicles recommend thinner oils not only for mileage and quicker circulation, but OEM tolerences and ring seals are better these days, so thicker oils are not needed.