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Oil weight, or viscosity, refers to how thick or thin the oil Is.
The temperature requirements set for oil by the Society of Automotive
Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).
Oils meeting the SAE's low temperature requirements have a "W" after
the viscosity rating (example: 10W), and oils that meet the high
ratings have no letter (example SAE 30). An oil is rated for
viscosity by heating it to a specified temperature, and then allowing
it to flow out of a specifically sized hole. Its viscosity rating is
determined by the length of time it takes to flow out of the hole. If
it flows quickly, it gets a low rating. If it flows slowly, it gets a
high rating.
Engines need oil that is thin enough for cold starts, and thick
enough when the engine is hot. Since oil gets thinner when heated,
and thicker when cooled, most of us use what are called multi-grade,
or multi-viscosity oils. These oils meet SAE specifications for the
low temperature requirements of a light oil and the high temperature
requirements of a heavy oil. You will hear them referred to as
multi-viscosity, all-season and all-weather oils.
When choosing oil, always follow the manufacturer's recommendation.
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