The numbers relate to the viscosity of the oil. 0 means that at low temperatures, the oil is still very thin which is what you want. Cold starting needs thin oil to circulate through the engine and prevent wear, so this is good.
The 30 or 40 number is the viscosity of oil in high temperatures, so at freeway speeds, the oil is thick enough to provide protection for your hot engine.
Now, the two sets of numbers mean that at low to high temps, your oil will protect your engine well. The oil is specially formulated to modify itself as temperature increases, that is at cold temps the molecules are short chains, then when engine / air temps rise, the molecules bond to themselves to form longer chains thus protecting the engine.
The specs in the manual, are not as good as 0 - 40 oil, so you are using a higher grade of oil than Hy specify, nothing wrong with that at all.
In Australia, unless you live in the High country, a 0 grade oil is a bit unnecessary. My Diesel is currently running 5 - 40 oil.
Hope this helps.
