Its just hot viscosity, at working temps a w40 oil will remain slightly thicker/more viscus that a w30 oil, therefore will have slightly more resistance to shear and will be slightly better at providing a slightly higher oil pressure, so better for a worn engine.
I will add that its only a slight difference, but it helps on a high mileage engine, Nissan and most other manufactures recommend a 5 or 10W30 oil mainly for fuel economy (less oil drag), and the engines are designed around metals that will allow running thinner oils.
Oh! and the first figure IE 5 or 10 is the cold viscosity, and can be pretty much ignored in our climate.
great i always wanted to know what the numbers related to tbh . cheers iv got a lot better understanding what to go for now