These numbers are the viscosity index which is defined in tribology charts. These numbers ensure you to have consistent oil at all possible temperatures.

W is winter for sure. If you are not living in an African country or in Russia or Norway, you don't need to be careful about the numbers. Any product will be fine. If you know the exact numbers suggested by the manufacturer, you should follow them of course...

Don't hesitate to use any kind oil if there is a leak or something which cause running out oil.