good but
5w20
Depends on year and engine size and climate, could be 5w20, 5w30, 10w30, or 15w40Depends on year and engine size and climate, could be 5w20, 5w30, 10w30, or 15w40
4.75qt of oil 5w20
5W20.
When the vehicle manufacturer recommends it.
Even if you can it's not a good idea. These oils have different viscosity and might have different chemical content. When you mix the oils it can change their mechanical properties and the oil will not be as good as when they are separate. You can get special additives to tune 5w20 in something in between 5w20 and 5w30.
5w20 or 5w30
It depends on the situation. Personally, I wouldn't use 5w20 instead of 10w30 for a complete oil change. However, using it to top off a quart or two shouldn't be an issue. Oil shears over its life so a little thinner oil shouldn't hurt the engine.
7 quarts 5w20
4.5 quarts 5w20
5 quarts usually 5W20 oil.
5w20