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.
Viscosity ratings for commonly used motor oils typically range from 0 up to 50. With multi-viscosity oils, a "W" after the number stands for "Winter" grade oil. The numeric value of the first number (example 5W-20) is a measure of the pour point of the oil expressed in degrees Celsius whent he oil is cold. The rating is determined in a lab using a cold crank simulator and mini-rotary viscometer test. The oil weight is its viscosity index at 100 degrees C (the boiling point of water).
Low viscosity motor oils that pour easily at low temperatures typically have a "0W", "5W" or "10W" rating. There are also "15W" and "20W" grade multi-weight motor oils.
Higher viscosity motor oils that are thicker and better suited for high temperature operation. These may be multi-grade oils or single weight oils such as SAE 30, 40 or 50.
Single weight 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.
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 SAE 10W 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 0W-20, 0W-40, 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 rating, while the second number refers to the oil's high temperature viscosity rating.
Note: Motor oils that have a wider range viscosity rating such a 5W-30, 5W-40 and 0W-40 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 0W-20 or 5W-20 will contain a higher percentage of thinner base stock oil (which is typically a synthetic oil). This requires more viscosity improver additive to achieve a the same high temperature rating as a 10W-30, 10W-40 or straight 30 or 40 weight oil.
Most vehicle manufacturers today specify 5W-20 or 5W-30 for newer vehicles for year-round driving. Some European car makes also specify 0W-20, 0W-30, 0W-40 or 5W-40 for their vehicles. Always refer to the vehicle owners manual for specific oil viscosity recommendations, or markings on the oil filler cap or dipstick.
Always use the motor oil viscosity recommended by your vehicle manufacturer. Using a different viscosity (thinner or thicker) may cause oil pressure and oil supply problems, especially in late model engines with cylinder deactivation and/or variable valve timing (VVT).
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, typically specify 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 0W-20, 0W-30, 5W-20 or 5W-30 to reduce friction and drag.
For 2011, General Motors has announced a new oil requirement called "dexos." GM says their new oil performance specification is better than the new GF-5 specification, which also goes into effect this fall. GM says dexos will be required in all 2011 and newer GM engines, and it will be backwards compatible with older engines that use SM oils.
There will be two versions of dexos: dexos1 for gasoline engines and dexos2 for diesel engines. The specification calls for a high quality synthetic base stock with additives that provide high temperature, high sheer characteristics to reduce friction for better fuel economy, to reduce piston ring deposits and sludge, and to extend oil life (necessary for use with GM's Oil Life Reminder System).
Because it uses high quality synthetic base stocks, dexos and other brands of oil that meet GM's dexos specification will likely cost 30 to 60% more than conventional motor oil. GM will license other brands that meet their spec. Pennzoil Platinum and Quaker State Ultimate Durability both claim to meet the new dexos spec now in their SAE 5W-30 viscosity grade motor oils.
Following an investigation by the state attorney general's office, New Jersey officials have banned the sale of 19 motor oil products for deceptive labeling. For details, Click Here. The banned products were found to have viscosity ratings that varied widely from those marked on the product. All of the banned oils were tested by an independent lab contracted by the state. The tests found that each of the banned products failed to meet the viscosity measurements specified on their labels.
Motor oils are graded by viscosity. Using a motor oil with a viscosity grade that does not meet the requirements of a particular car engine can lead to a decrease in gas efficiency and inreased engine wear and other problems.
Many of the banned products were made to appeal to thrifty consumers, with sale prices ranging from $3 to $5 per quart. Most were sold at grocery stores, gas stations and discount retailers as alternatives to more expensive brand name motor oils.
New Jersey is the sixth state to ban all or some of the 19 motor oils from sale. The banned products are:
Auto Club Motor Oil SAE 5W-30
Auto Club Motor Oil SAE 10W-30
Auto Club Motor Oil SAE 10W-40
Auto Club Motor Oil SAE 20W-50
Black Knight Motor Oil 5-30
Black Knight Motor Oil 10-40
LubeState Motor Oil SAE 10W-30
MaxiGuard MG 10-30
MaxiGuard MG 10-40
MaxiGuard MG 20-50
MaxiGuard MG SAE30
Orbit Motor Oil 5-20
Orbit Motor Oil 10-40
TruStar Motor Oil 10-30
U.S. Economy Motor Oil 5-30
U.S. Economy Motor Oil 10-40
U.S. Economy Motor Oil SAE 10W-40
U.S. Spirit Motor Oil SAE 10W-30
U.S. Spirit Motor Oil SAE 10W-40
Consumers who have purchased any of these banned products should return the oil to the point of purchase and request a full refund.