break in oil and changes

correct me if Im wrong, but isn't a multi viscosity oil like 5w 20 or 10w 30 suitable for multi temperatures and the 5w or 10w refers to the cold weather viscosity, lower the W number the thinner at cold temps? So SAE 30 would be the same at any temp? And break in oil has zinc and phosphorus addivitves for break in. Or is there a difference between conventional oil and break in oils....?