Viscosity measures the resistance to flow. Higher viscosity grades have more resistance to flow than lower viscosity grades. Oil gets thinner as it gets hotter. To determine the correct viscosity for an application you need to know the operating temperature of the oil in that application.
Engines that run high operating oil temperatures require higher viscosity oil. Engines that run low oil temps require lower viscosity oil. The number before the “W” reflects the “Winter” cold start flow of the oil. A lower number before the “W” indicates better cold start protection (lower viscosity base oil). The number after the “W” indicates the flow rate at 212F (high temperature viscosity). Crankcase oils and gear oils are based on 100 Deg C viscosity (212 Deg F). So a 10W-30 weight oil, means that the oil is a 10 weight oil that will not thin more than a 30 weight would when hot. Many people run slightly lighter or lower viscosity oil in the winter for better cold start ability and a heavier and more viscous oil in the summer for better protection in heat. Check with your owners manual for the proper oil for your machine.
Do you have a tech question you want GearHead to answer? Email your question to firstname.lastname@example.org