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What You Should Know About Motor Oil Viscosity
by Larry Carley copyright AA1Car.com (Updated 1-23-2026)
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.
In recent years, there has been an industry-wide trend to thinner, lower viscosity oils such as 0W-20. This has been the result of government mandated fuel economy requirements.
A lower oil viscosity motor oil flows more easily when the oil is cold (more like water than molasses). This allows the oil to reach the upper valvetrain components and camshaft(s) in OHC engines more quickly to provide much needed lubrication. The thinner viscosity oil also reduces internal friction and drag, allowing easier cranking when the engine is cold. A thinner oil also reduces overall friction which helps improve performance.
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.
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 have not been used in automotive gasoline engines since the 1960s, but they may still be specified for small air-cooled engines such as lawn mowers, chain saws, portable generators and stationary pumps.
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. Also, an oil with a lower winter rating like 0W-20, oW-40 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 0W-20 for newer vehicles for year-round driving. However, auto makers may also specify 0W-40, 5W-20 or other viscosities depending on the application.
Always refer to your owners manual for the recommended oil viscosity for your engine. You may also find the type of oil specified on the engine oil filler cap or dipstick.
Why use the 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 0W-20, 0W-40 or 5W-20 to speed lubrication of the overhead cam(s) and valve-train when the engine is first started. Pushrod engines, by comparison, may also specify the same 0W-20 or 0W-40 oils, or a heavier 5W-30, 10W-30 or 10W-40. Again, refer to your owners manual to see what uis recommended
Some auto makers have been experiencing premature engine failures with certain late model engines. These failure have been blamed on several factors, including not properly cleaning engine blocks and crankshafts during manufacturing (residual debris damages the bearings), not maintaining required bearing clearances during engine assembly (too loose or too tight), and using 0W-20 oils in applications that should probably use a thicker oil like a 5W-20 or 0W-40 (such as in hard-working pickup trucks used for towing, or high performance turbocharged or supercharged engines). Consequently, GM, Ford, Toyota and others have revised some of their original factory oil recommendations and now specify a higher viscosity oils for certain engines and applications.
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 0W-20 now has 100,000 miles on it, switching to a thicker 5W-20, 5W-30 or 0W-40 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.
Back in 2011, General Motors announced a new oil requirement called "dexos." GM said their new oil performance specification is better than the GF-5 specification, which also went into effect the same year. GM said dexos is required in all 2011 and newer GM engines, and it is backwards compatible with older engines that use SM oils.
There are 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).
GM has licensed various oil companies to produce dexos motor oil under their own brand names, so check the label on the product to see if it meets the specifications.
The newest ILSAC specifications released in 2025 is GF-7, which is for certain engines that require a untra-low viscosity motor oil such as 0W-10 or 0W-12. This new rating is only for specific late model engines, and is NOT backwards compatible with older engines or engines not specifically designed for these new next generation super thin oils.
The current ILSAC GF-6 motor oil specification has been around since 2020, and is similar to the API SP specification. GF-6 is formulated to meet the demanding lubrication requirements of late model today's smaller turbocharged engines, and those with Direct Gasoline Injection (GDI). These type of engines require increased protection against Low-Speed Pre-Ignition (LSPI) and timing chain wear. ILSAC GF-6 motor oils meet the requirements of modern engines better than GF-5 oils. The GF-6 oils also provide better slightly fuel economy.
The GF-6 standard is split into two sub-categories. The distinction between the two is as follows:
GF-6A: Backward-compatible standard for SAE 0W-20, SAE 5W-20, SAE 0W-30, SAE 5W-30, and SAE 10W-30, but NOT SAE 0W-16. This standard continues to utilize the ILSAC starburst symbol for licensing.
GF-6B: New standard for SAE 0W-16 ONLY, developed to meet the specific needs of modern engines. The 0W-16 viscosity is intended to provide a significant improvement in fuel economy for engines
designed to use this oil. Th 0W-16 viscosity is NOT be backward-compatible to older engines and will have a new licensing symbol for the bottle.
The engine tests necessary to meet the ILSAC GF-6A and GF-6B standards include eight separate tests (seveno of which are new for these oils). The tests include:
Sequence IIIH – oxidation and deposits
Sequence IVB – valve train wear
Sequence V – sludge and varnish
Sequence VIE – fuel economy
Sequence VIF – 0W-16 fuel economy
Sequence VIII – corrosion (not required for GF-6B)
Sequence IX – LSPI
Sequence X – timing chain wear
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