The majority of the drivers will look at the numbers on a label on the engine oil like 5W-30 or 10W-40 and assume that the only reason is the picture of how the oil will operate. Nevertheless, the most important difference that individuals can overlook is the difference in grade of viscosity (the SAE number) that describes the viscosity of the oil at a specific constant temperature, and the V.I. that can be used to interpret how viscosity will change in case the temperature falls or becomes higher in the process of driving along the normal course.
Viscosity index is one of the terms that can be used to indicate to what extent an engine oil is capable of enduring alteration in the temperature, therefore it is a crucial factor as far as the protection of an engine is concerned in reality. The issue with an engine oil that lacks the viscosity-stability is that it can be excessively viscous during the start-up but excessively thin during the operating temperature, therefore it is most likely to wear. Knowledge of VI may be applied to determine how some oils would react to changes of temperature compared to others thereby getting to be more reliable on how they would protect themselves when the mornings were colder, or when on the highway rushing.
In the event that you have a selection of car oils to use, you might want to explore our engine oil solutions for different vehicles can find a formulation that takes these performance variables into consideration.
What Is Viscosity Index and Why It Exists
The viscosity index is an index, a dimensionless ratio, and is a measure of the effect of temperature on the viscosity (resistance to flow, or thickness) of an oil. The higher the VI, the thinner becomes the oil at hot or the thicker becomes the oil that is at cold.
Engine oils either become thin because of rising of temperature- this is elementary physics. At low temperatures, the molecules are moving slower and contain a greater amount of resistance that makes the oil additionally feel thicker. As the engine is operating, the molecules are not bound to any substance thus becoming less viscous when it gets hot. In this way, only the radical change can be achieved without the sound stability of temperature and leads to the poor lubrication on both sides.
The concept of viscosity index was developed in the end of 1920s, when the concept was developed by engineers at Standard Oil (which was subsequently purchased by ExxonMobil). As lubricants, they principally used single-grade oil, and the drivers had to alternate between the so-called winter and summer grade as the viscosity varied widely season to season. VI scale was meant to draw comparisons among oils and reference standards: one with low- VI naphthenic oil (VI=0), the Vi that varied radically, and the other with high- VI paraffinic (VI=100) that was relatively constant. This facilitated better defects of temperature characteristics and preconditioned multi-grade oils.
Viscosity Index vs SAE Viscosity Grade — Key Differences
These two terms are too simple to interchange them as the two are dealing with the thickness of the oil whichever way they are measuring two completely different things.
The Viscosity Index is linked with stability in the wide scope of the temperature and SAE viscosity grade includes the attributes of the oil flow at specified cold and hot test points.
Here’s a clear comparison:
| Parameter | Viscosity Index | SAE Viscosity Grade |
| What it measures | Stability and temperature Viscosity. | Oil viscosity at constant temperatures. |
| Unit | Index number (e.g., 150, 180) | Grade (e.g., 5W-30, 10W-40) |
| Main purpose | Performance consistency | Cold start and characteristic operating thickness. |
To get more information,check out the difference between 5W-30 and 10W-40.
How Temperature Changes Affect Engine Oil Behavior
Temperature swings are among the biggest problems of the engine oil when it is used in daily use.
During cold weather, the oil is thick and therefore flows slowly thus the problem with cold start is hard to start an engine. Sluggish flow means that it takes longer periods to lubricate critical parts like bearings and camshafts that have more and more metal-to-metal contacts and wear. At the other end, depending on the operating temperature of the engine (usually 90 -110 C loads above this temperature), the oil is very thin. Once it is too thin the protective film becomes weak and it might be left to the chance of being overworked with heavy-weight or scheduled to travel a good many miles in the day.
The result of these extremes is a stress: the oil must pump freely when cold, and must possess sufficient body when hot, to hold parts that are moving asunder. The problem is even larger because of extensive daily or seasonal variations of temperature and oil having elevated temperature sensitivity is more certain.
To learn more about these dynamics, read our guide on how temperature affects engine oil performance.
Why High Viscosity Index Oils Perform Better in Real-World Driving
High index viscosity oil could hold comparatively constant oil viscosity within broad temperatures and this indicates that it defends better in usual driving situations.
Most drivers undergo varied situations: early in the mornings they might experience cold start and there might be traffic that is handicapped and makes the oil gradually warm as well as road traveling that is in constant hot temperatures, and on towing the drivers might experience enormous loads. High-VI oil will not thin significantly under operation temperature, and offers film strength to give a better wear protection. It also tends to get too thick when in cold environments so as to move faster and less startup wears are encountered.
In real-world conditions, with non-uniform operating temperatures of engines (typical of synthetics and good mineral oils), those with a VI higher than 150 are smoother and the engines have lower friction loss and extended protection durations than those with lower VI oils that do not run consistently.
Our article on viscosity index and engine oil performance will help you see our article on viscosity index and engine oil performance.
How Viscosity Index Influences Oil Stability Over Time
The viscosity index is also significant in the fresh oil composition, as well as, the degree to which the oil would sustain itself in the course of its usage.
High VI oils are also likely to have better base stocks and with shear-stability, viscosity index modifiers (polymers that swell when temperature rises). These prevent mechanical shear (cutting of the parts of the engine machinery using the oil) and oxidation (dissolution by the heat and air exposure) in the interest of ensuring the viscosity is continued.
This type of stability provides it with higher drain interval as it does not easily deteriorate to a smaller form that is less protective. The lower-VI formulations are able to shear thus providing less protection over time.
Since viscosity index can influence the stability of oil, how viscosity index affects oil stability.
Common Misunderstandings About Viscosity Index
Still, there are two myths about VI that can make them confused.
To start with, good oil is not necessarily better. Although a higher viscosity is usually recommended when it comes to stability of the temperature in a base oil, VI must find a balance with other qualities like quality of the base oil, additive package/ shear resistance. Large VI due to widespread application of polymer may prevent the integrity of strength in the long run in case the improvers are destroyed.
Second, it is an opinion of others that VI is a substitute of viscosity grade – it is not. The SAE grade is most significant specifications of the cold and hot performance and VI is applied in order to offer continuity between both specifications.
Third, VI is not germane in deep climate- no. Even the good stability of intermediate daily swings (e.g. 10 o C in the mornings and 35 o C afternoons) are favorable, and all modern engines are run on load at a higher temperature, regardless of location.
How Drivers Should Use Viscosity Index When Choosing Engine Oil
Start with the suggestion of your car company, it should as safe as you can get.
VI is especially important in unstable weather conditions of travel, when there is a large number of cold starts, when it is necessary to carry large loads, when the time of replacing the oil filter decreases. The larger the VI (usually 140 or higher) of the oils, the more consistent are the oils, especially the multi-grade synthetics or good conventional ones.
Buyer beware: trade off with your terms: where it is always hot, a minor decrease in VI can be acceptable as long as the grade is capable of meeting the demand, where conditions are mixed, or cold, more VI will be dependable in start up and protection.
Reference oils product sheet- VI reputable oils and other specifications.
Conclusion — Viscosity Index Explains Real-World Oil Behavior
In essence, viscosity index shows how an engine oil can retain its protective properties in conditions of change of temperature, a literal test of metal in extreme conditions, which is not the case in the laboratory.
The requirement of the daily driving supposes cold-to-hot shifts, and the fact that some oils can grant a person the best protection and the least number of issues in the long-term perspective can be explained by the knowledge of VI. It turns laboratory specifications and road measurements and makes more intelligent choices related to engine life.As the formulations have changed the focus has been made in the stability and overall performance through careful selection of the base oils used and application of technology of additives.