Viscosity

Summary
Viscosity rating indicates flow rate of oil at a given temperature. There are many grades. Thin oils tend to be for cold conditions. Oil with improver is called multi-grade or multiple-viscosity oil.

For oil to do all of the work that’s expected of it, it must have special properties.

Its viscosity is crucial. Viscosity is a measure of how easily a liquid flows. Low-viscosity liquid is thin and flows easily. High-viscosity liquid is thick and flows slowly.

Lubricating oil must be thin enough to circulate easily between moving parts, but not so thin that it will be forced out from between them.If it is forced out, parts will be left in direct contact and they’ll be damaged.

If it’s too viscous, it moves too slowly to protect parts, especially in a cold engine.

It was once normal practice for engines to need one grade of oil for summer, and another for winter. Oils are graded or classified by the American Society of Automotive Engineers, or SAE. An engine oil with an SAE number of 50 has a higher viscosity, or is thicker, than an SAE 20 oil. Oils with low viscosity ratings, say SAE 5W, 10W, and 20W were tested at a low temperature, around minus 18 degrees Celsius, and were used for cold conditions. Oils with high viscosity ratings, say, SAE 20, 30, 40 and 50, were tested at a high temperature, around 99 degrees Celsius, and were used for hotter conditions. Modern oils however are blends of oils which combine these properties. The oils are blended with additives, called viscosity index improvers, to form multi-grade, or multi-viscosity oils. They provide better lubrication, over a wider range of climatic conditions than monograde oils.

Oil is also classified by the American Petroleum Institute or API service classification. Oils for spark-ignition engines carry a prefix S, and diesel or compression-ignition engines use C. Some oils have additives that make them suitable for both. Manufacturers recommend which viscosity and API classification is ideal for a particular engine.

reference
CDX