It’s simple.
The number before the W is the Winter weight of the oil.
So the oil will flow like the weight of that oil down to zero degrees, so it doesn’t get too thick and starve the engine of oil during a cold start.
The number after the W is the weight of the oil once the engine reaches operating temperature.
All the small engines I run (summer/winter) have 5W30 oil in them.
Tester