The number before the W indicates the oil flow during cold engine starts
The lower this number the faster the oil will flow and immediately lubricate the engine parts resulting in better protection of engine parts
The number after the W indicates the oil performance at high temprature
Means k the higher this number the oil will have no problems lubricating at higher temprature
So if you are in a snow country you will not use a 20W-50 or 10W-40 rather you will use 0W-30 or 5W-30/40
And in a desert you will be using a 15W-40/50 or 20W-50/40 irrespective if its mineral or synthetic