Oil viscosity refers to how easily oil pours at a specified temperature. Thinner oils have a water-like consistency and pour more easily at low temperatures than heavier, thicker oils that have a more honey-like consistency. Thin is good for easier cold weather starting and reducing friction, while thick is better for maintaining film strength and oil pressure at high temperatures and loads.
The viscosity rating of a motor oil is determined in a laboratory by a Society of Automotive Engineers (SAE) test procedure. The viscosity of the oil is measured and given a number, which some people also refer to as the "weight" (thickness) of the oil. The lower the viscosity rating or weight, the thinner the oil. The higher the viscosity rating, the thicker the oil.
MOTOR OIL VISCOSITY RATINGS
Viscosity ratings for commonly used motor oils typically range from 0 up to 50. A "W" after the number stands for "Winter" grade oil, and represents the oil's viscosity at zero degrees F.
Low viscosity motor oils that pour easily at low temperatures typically have a "5W" or "10W" rating. There are also 15W and 20W grade motor oils.
Higher viscosity motor oils that are thicker and better suited for high temperature operation typically have an SAE 30, 40 or even 50 grade rating.
These numbers, by the way, are for "single" or "straight" weight oils. Such oils are no longer used in late model automotive engines but may be required for use in some vintage and antique engines. Straight SAE 30 oil is often specified for small air-cooled engines in lawnmowers, garden tractors, portable generators and gas-powered chain saws.
MULTI-VISCOSITY MOTOR OILS
Most modern motor oils are formulated from various grades of oil so the oil will have the best characteristics of both thick and thin viscosity oils. Multi-viscosity oils flow well at low temperature for easier starting yet retain enough thickness and film strength at high temperature to provide adequate film strength and lubrication.
A thin oil such as a straight 10W or even a 20W oil designed for cold weather use would probably not provide adequate lubrication for hot weather, high speed driving. Likewise, a thicker high temperature oil such as SAE 30 or 40 would probably become so stiff at sub-zero temperatures the engine might not crank fast enough to start.
Multi-viscosity grade oils have a wide viscosity range which is indicated by a two-number rating. Popular multi-viscosity grades today include 5W-20, 5W-30, 10W-30, 10W-40 and 20W-50. The first number with the "W" refers to the oil's cold temperature viscosity, while the second number refers to its high temperature viscosity.
Note: Motor oils that have a wider range viscosity rating such a 5W-30 are blended with more base stocks and additives. Because of this, it may be harder for a wider range oil to remain in grade as the miles accumulate (which is why GM does NOT recommend using 10W-40 motor oil -- they say it breaks down too quickly and does not say in grade as long as 10W-30 or 5W-30). Also, an oil with a lower winter rating like 5W-30 or 0W-30 will contain a higher percentage of thinner base stock oil. This requires more viscosity improver additive to achieve a the same high temperature rating of a 10W-30 or straight 30 oil.
Most vehicle manufacturers today specify 5W-30 or 10W-30 motor oil for year-round driving. Some also specify 5W-20. Always refer to the vehicle owners manual for specific oil viscosity recommendations, or markings on the oil filler cap or dipstick.
As a rule, overhead cam (OHC) engines typically require thinner oils such as 5W-30 or 5W-20 to speed lubrication of the overhead cam(s) and valve-train when the engine is first started. Pushrod engines, by comparison, can use either 5W-30, 10W-30 or 10W-40.
As mileage adds up and internal engine wear increases bearing clearances, it may be wise to switch to a slightly higher viscosity rating to prolong engine life, reduce noise and oil consumption. For example, if an engine originally factory-filled with 5W-30 now has 90,000 miles on it, switching to a 10W-30 oil may provide better lubrication and protection. The thicker oil will maintain the strength of the oil film in the bearings better so the engine will have more oil pressure. This will also reduce engine noise and reduced bearing fatigue (which can lead to bearing failure in high mileage engines).
For sustained high temperature, high load operation, an even heavier oil may be used in some situations. Some racing engines use 20W-50, but this would only be recommended for an engine with increased bearing clearances. Increasing the viscosity of the oil also increases drag and friction, which can sap horsepower from the crankshaft. That's why 20W-50 racing oil would not be the best choice for everyday driving or cold weather operation for most vehicles. The latest trend in racing is to run tighter bearing clearances and use thinner oils such as 5W-30 or 5W-20 to reduce friction and drag.
By Larry Carley c2006 http://www.aa1car.com/library/oil_viscosity.htm
