What Is Multi-Grade Engine Oil?
The Society of Automotive Engineers is a professional organization and standards body for the engineering of powered vehicles of all kinds - cars, trucks, boats, aircraft and more.
The SAE designation for multi-grade oils includes two grade numbers; for example,10W-30 is the designation for a common multi-grade oil. The first number associated with the W, the 10, means that the viscosity of this multi-grade oil at winter temperature corresponds to the viscosity of a single-grade 10W oil at winter temperature.
The second number, 30, means that the viscosity of this multi-grade oil at hot operating temperature corresponds to the viscosity of a single-grade 30 oil at the operating temperature. The motor oil grade to be used in a given vehicle is specified by the manufacturer of the vehicle.
VISCOSITY is the resistance to flow of a fluid. As an example, water flows easily so it will have a low viscosity [a low resistance to flow] while cooking oil has a high viscosity [a high resistance to flow]
Here's The Bible On Motor Oils