Oil viscosity is the measure of oil's ability to resist flowing. The Society of Automotive Engineers (SAE) has set low and high temperature requirements for oil.
Low = 0 degrees Fahrenheit or –18 degrees Celsius
High = 210 degrees Fahrenheit or 99 degrees Celsius
Oils that meet the SAE low temperature requirements have the letter W (stands for winter, not weight) following the viscosity rating (SAE 10w).
Oils that meet the high temperature requirements don't have the "w" after the SAE 30.
Oils thin out when heated and thicken when cooled. A good engine oil must be thin for cold starts and thick enough at high temperatures.
This is where multi-grade oils come into play.
The weights given on oils are arbitrary numbers assigned by the SAE. These numbers correspond to "real" viscosity, as measured by several accepted techniques. These measurements are taken at specific temperatures.
Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50.
The "w" means the oil meets specifications for viscosity at various low temperatures depending on weight, and is therefore suitable for Winter use.
5W is tested at -25C
10W at -20C
15W at -15C
20W at -10C
Use the chart below to familiarize yourself with the chemical and physical properties of conventional motor oil: