The viscosity grade like 0W-20 or 10W-40 is used to show how the engine oils act in cold start and in working temperature, rather than being used to indicate the effectiveness and quality of the oil itself. A number like 0W- 20 or 5 W 30 would indicate the behavior of engine oil at various temperatures and not the strength or quality of the oil.
Most motorists think that the higher the second number the better protection to the engine is provided and that a heavier oil is always better. In reality these viscosity figures characterize the flowing behavior of the oil over a broad temperature spectrum- essential to the pumpability of the oil when cold and the strength of the oil film when hot.
After many years of experience in the lubricant formulation and engine test data area, I have noticed that incorrect interpretation of these numbers will often result in incorrectly matched oil selections, which may either cause increased wear or less efficiency.
Breaking Down the Numbers in Engine Oil Grades
Each multigrade engine oil label is based on the same logic of SAE J300: the first number (and W) is used to denote low-temperature performance, and the second one is used to denote high-temperature viscosity.
The W means winter, and it indicates that the grade has been subjected to cold-flow. A lower value preceding the W indicates that the oil is fluid at a lower temperature, and as a result, it undergoes less cranking resistance, and the oil flows better in the engine during starter.
The second figure is the kinematic viscosity of the oil at 100 o C (and frequently, the viscosity at high temperature, when the engine is fully warmed) which determines the viscosity of the oil at the highest temperature reach by the engine.
| Grade Component | What It Represents |
| First number (e.g. 0W) | Low-temperature flow performance |
| “W” | Winter rating |
| Second number (e.g. 20, 30, 40) | High-temperature viscosity |
Such two specifications enable the use of one oil to work in seasonal temperature changes- something that could not be effectively done by single-grade oils.
What Does 0W-20 Mean?
0W-20 is designed to have excellent cold-start fluidity with a low intentionally set operating viscosity.
In the 0W rating, the cranking and pumping viscosity is very low at the very cold temperatures (down to about -35 o C), and rapid flow of oil is possible even in the worst winter conditions. The operating temperature of 20 indicates that the oil is kept in a comparatively thin film, so internal friction is minimized and fuel economy goals of the day are achieved.
| Feature | 0W-20 Characteristics |
| Cold flow | Very strong |
| Operating viscosity | Low |
| Typical application | Modern passenger cars |
This grade has become the factory-fill grade on most of the more recent gasoline engines that have tight bearer clearance and variable valve timing (VVT) systems optimized to use lower-viscosity oils.
What Does 5W-30 Mean?
5W-30 offers a reasonable compromise between high-temperature high-load application and sufficient cold-weather operation.
The 5W section of the formula allows a dependable flow and pumpability in fairly cold climate (often tested down to approximately -30 C ) and the 30 grade grade produces a more substantial oil film in the operating temperature than the 20 weight oils. This is an excellent all-season mixture that is not too draggy.
| Feature | 5W-30 Characteristics |
| Cold performance | Good |
| High-temp protection | Balanced |
| Typical application | Wide range of passenger cars |
Over many decades, the 5W-30 has been a commonly used specification in passenger cars in temperate climate, with a very broad range of engine designs and driving patterns.
What Does 10W-40 Mean?
10W-40 offers a better high temperature viscosity and makes a modest trade off in low temperature performance.
The 10W rating provides only fair startup performance in cooler and not extreme winter operation, and the 40 rating in operating temperature provides an observably heavier oil film – useful to keep the separation intact under heavy loads or high temperature conditions.
| Feature | 10W-40 Characteristics |
| Cold performance | Moderate |
| High-temp viscosity | Higher |
| Typical application | Older engines or warmer regions |
This grade is still widely used in warmer climates, with older engines with broader clearances, or where high temperatures and loads are frequent.
You can explore various common engine oil viscosity grades to find options that align with these performance characteristics.
How to Compare These Viscosity Grades in Real Driving Conditions
The real practical importance of each of the grades is made evident when they are compared to the real operating conditions and engine demands.
In extremely cold climates or during short distance urban commuting, 0W- 20 reduces the wear during startup by circulating rapidly until the engine is operating again. In continuously hot weather, long highway travel, or loosely-toleranced older engines, 10W-40 has a better film thickness under thermal load.
In many cases 5W-30 is a good compromise, because it is good in moderate cold and moderate hot conditions, and it fits most naturally aspirated and most turbocharged passenger car engines.
The choice of these grades is decided by the engine manufacturers depending on the design of the bearing, oil pump capacity, tension in the piston ring and fuel economy regulations. The non-conformance to the recommended grade may change the efficiency of hydrodynamic lubrication, raise the parasitic losses, or hasten the wear of components.
Common Misinterpretations of Oil Grade Numbers
- Clogs provide greater protection of the engine at all times– Thick oil has better protection- Thick oil does provide greater resistance to the fluid, raises oil temperatures, and in cold starting could be delayed in reaching other parts of the engine, possibly resulting in more wear than the rightly chosen lower-viscosity grade.
- A single viscosity grade suits any climate – an A grade suited to tropical heat can lead to excessive cranking drag and in winter can provide insufficient circulation, and a very low-viscosity grade can be washed away in a long, hard run at high temperature.
- Oil grade= oil quality– SAE viscosity grade does not state the quality of the base oil, 2 performance of the additives, oxidation stability, or deposit control only the flow behavior at the given temperature.
Such misconceptions are often based on old-fashioned ideas about single-grade oils and not taking into account the temperature-specificity of the modern multigrade formulations.
Conclusion — Oil Grade Numbers Describe Behavior, Not Quality
The figures in 0W-20, 5W-30 and 10W-40 are standardized values of the behavior of engine oil when cold starting and at normal operating temperature.Proper reading of engine oil viscosity values will avoid improper choices that may interfere with lubrication. Through the correlation of the grade to the specification of the engine manufacturer and the current operating environment, the constant film strength, low friction and dependable long term engine protection are attainable.