Select the Right Oil: A Simple Guide to Understanding Lubricant Viscosity
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In lubrication, it is fundamental to understand the viscosity of a lubricant, which provides the critical basis for selecting the right lubricants, no matter precise devices or heavy equipment. It is so important to ensure your machinery operates smoothly for a longer life and works effectively.
What is the viscosity of oil?
Viscosity is the resistance of a fluid to flow. Simply speaking, A lubricant's viscosity indicates the oil's thickness level. A higher viscosity oil is thicker and flows more slowly, while a lower viscosity oil is thinner and flows more easily.
e.g., the viscosity of water @ 20°C is approx 1 cSt, but honey has a viscosity of approx 12000 @ 20°C.
The following table shows the viscosity of common liquids.
| Viscosity of Common Liquids | ||
| Liquid | Approx. Viscosity @ 20°C (cSt) | Approx. Viscosity @ 40°C (cSt) |
| Gasoline | 0.7 | 0.5 |
| Water | 1 | 0.66 |
| Milk | 2.5 | 1.5 |
| Olive Oil | 80 | 40 |
| Canola Oil | 75 | 42 |
| Hydraulic Oil (ISO VG 46) | 128 | 46 |
| Engine Oil (10W-30) | 300 | 70 |
| Shampoo | 5000 | 1500 |
| Honey | 12000 | 2000 |
For lubricants, viscosity is a super-important parameter. It's so important that we often name the oil after it. A great example is hydraulic oil "ISO VG 46" – that "46" comes directly from its viscosity measured at 40°C.
How is the viscosity grade of motor oil defined?
The SAE J300 standard defines the viscosity grade of engine oils. Taking "5W-40" as an example:
W: It means Withstand Winter, representing the oil's performance in cold weather.
Number before "W" (5W): The Low-temperature Performance grade. A smaller number means the oil is thinner in cold weather, making cold starts less difficult.
Number after W (40): The High-temperature Performance grade. A larger number means the oil is stickier at high temperatures, providing a stronger oil film.
The SAE J300 standard
| SAE Viscosity Grade | Low-Temp Cranking Viscosity (cP) Max | Low-Temp Pumping Viscosity (cP) Max (with no yield stress) | Kinematic Viscosity (cSt) at 100°C Min | Kinematic Viscosity (cSt) at 100°C Max | High Shear Viscosity (cP) at 150°C Min |
| 0W | 6200@ -35 | 60000@ -40 | 3.8 | - | - |
| 5W | 6600@ -30 | 60000@ -35 | 3.8 | - | - |
| 10W | 7000@ -25 | 60000@ -30 | 4.1 | - | - |
| 15W | 7000@ -20 | 60000@ -25 | 5.6 | - | - |
| 20W | 9500@ -15 | 60000@ -20 | 5.6 | - | - |
| 25W | 13000@ -10 | 60000@ -15 | 9.3 | - | - |
| 16 | - | - | 6.1 | <8.2 | 2.3 |
| 20 | - | - | 6.9 | <9.3 | 2.6 |
| 30 | - | - | 9.3 | <12.5 | 2.9 |
| 40 | - | - | 12.5 | <16.3 | 3.5* / 3.7** |
| 50 | - | - | 16.3 | <21.9 | 3.7 |
| 60 | - | - | 21.9 | <26.1 | 3.7 |
| *For 0W-40, 5W-40 and 10W-40 Grades ** For 15W-40, 20W-40, 25W-40 and 40 Grades |
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Viscosity Changes (ΔV) are also an important indicator of whether an oil change is necessary.
Generally speaking, if the kinematic viscosity of motor oil at 100°C changes by more than ±25% (a 25% increase or decrease in viscosity), it indicates it is time to change the engine oil.
As a lubricant approaches the end of its lifespan, its viscosity will first decrease markedly, then experience a swift rise. To avoid engine wear from lubrication breakdown, promptly changing the oil before the viscosity decreases notably is essential for ensuring proper vehicle performance
Motor oil turns out to be thicker beyond standard.
Thickening (viscosity increase of more than +25%) are often caused by oil oxidation, resulting in a large amount of sludge, leading to start-up wear and increased fuel consumption.
Motor oil turns out to be thinner than standard.
Thinning (viscosity decrease of more than -25%) is often caused by fuel dilution or additive ineffectiveness, leading to insufficent lubrication and severe wear.
How are viscosity grades of industrial oils defined?
The Standard ISO 3448 viscosity grade is the most universal and widely used classification standard for industrial lubricant viscosity, commonly used for hydraulic oils, gear oils, and other industrial oils.
ISO VG (Viscosity Grade), for example, ISO VG 32, ISO VG 68, ISO VG 220.
Core Definition: The number of each grade represents the center point of the kinematic viscosity of the oil product at 40°C. Its allowable fluctuation range are ±10%.
For example: The viscosity range of ISO VG 46 is 41.4 - 50.6 cSt (because 46 ± 10% = 41.4 ~ 50.6).
In the industrial lubrication field, viscosity changes are an important method for equipment maintenance. If a lubricant's viscosity shows significant up or down after a period of use, the maintenance engineers should pay immediate attention, as it may mean lubricant failure, which may cause serious equipment damage.
Oil Viscosity Increase:
Oil viscosity go-high always indicates severe oil oxidation, accumulation of sludge, or contamination with incompatible higher-viscosity oils.
e.g., In hydraulic systems operating under persistent high temperatures, the fluid oxidizes very rapidly, leading to viscosity increase. The operators should change new oil promptly.
If not, the aged oil will not only increase system energy consumption and cause sluggish response, but may also trigger serious failures like filter blockage.
Oil Viscosity Decrease:
There are 2 reasons why the lubricant viscosity drops: 1, contamination by lower-viscosity liquids, such as water or solvent ingress due to inadequate equipment sealing; 2, the molecular chains of viscosity index improvers were cut off by high mechanical shear.
Some low-cost gear oils on the market serve as a typical case: the manufacturer blended low-viscosity base oils with inexpensive polymeric thickeners. The look-good initial viscosity cannot withstand the intense shearing in the gear rotation. Their viscosity goes down rapidly within a short period, leading to insuficient oil film strength, and failing to lubricate, which causes equipment wear. This "cost-saving" approach actually significantly increases equipment failure risk, increases the maintenance cost, and shortens its service life.
Viscosity, Temperature and Viscosity Index Improvers (VIIs).
The viscosity is general measured at a specific temperature. The viscosity index (VI) measures how much a lubricant's viscosity changes with temperature. The higher the value, the less the value of viscosity changes with the temperature.
e.g.:
The VI of mineral oil (paraffinic-based) is approximately 96-110, while that of polyalphaolefins (PAO) can reach as high as 120-180. The VI of PAG (polyalkylene glycol) oil can reach over 190.
Because equipment and ambient temperatures fluctuate constantly, in most lubrication aplications, lubricants with high VI are preferred for wide temperature adaptability and more stable lubrication protection. For example, high-end automotive motor oils based on PAOs typically have a higher VI, better suited for cold start-ups and intense driving in hot temperatures.
How can the VI of a base oil be improved?
In addition to optimizing the base oil itself (physical refining and chemical conversion), lubricants with higher VIs can also be obtained by adding viscosity improvers. These lube additives are normally long-chain polymers that expand with heat, counteracting the natural thinning of the oil.
Viscosity index improver (VII)
Viscosity index improver (VII) is an additive used to formulate multigrade internal combustion engine oils and other high VI industrial lubricants. It increases the VI and reduces the change in viscosity with temperature in the lubricant formulations.
The most commonly used viscosity index improver is olefin copolymer (OCP), generally referring to ethylene propylene copolymer (EPM). OCP is one of the most widely used and mainstream VIIs in the world, especially in engine oils. There are also other VII, like PMA, HSD, please click here to know more about the viscosity index improvers.
General Introduction to Viscosity Index Improvers for Lubricant
2-side of viscosity: too high or too low
We should always select the lubricant at the proper viscosity grade according to the manual's instructions.
Automotive motor oil
- Viscosity too low
Too low a viscosity mainly results in insufficient lubrication and seal rings disabled, directly leading to engine wear and leakages.
- Viscosity too high
Excessively high viscosity means excessive resistance and poor flowability, which may result in the engine's difficult starting, high fuel consumption, and poor heat dissipation.
Industrial oils
Let's make an example with hydraulic oil; it may cause unnecessary problems if the hydraulic fluids are too high or too low beyond the standard.
- Viscosity too low
If the hydraulic fluids are too thick, this may increase pump resistance, hindering flow and potentially causing pump cavitation, which damages the pump. Furthermore, excessively high viscosity hydraulic oil increases flow resistance in the hydraulic system, increasing power consumption, slowing the response of actuators, and reduce accuracy.
- Viscosity too high
If the hydraulic fluids are too thin, the oil film becomes thinner, which results in Insufficient lubrication and increasing wear. Low viscosity oil also impairs sealing performance, increasing the risk of leakage.
What viscosities are tested for lubricants?
For Motor oils
- 100°C kinematic viscosity - under normal engine operating conditions.
KV100 determines the thickness and strength of the oil film during normal engine operation. For each viscosity grade, SAE J300 defines a minimum and optional maximum kinematic viscosity at 100°C.
- High Temperature High Shear (HTHS) viscosity - at 150°C - under severe engine operating conditions.
SAE J300 specifies minimum HTHS values for different viscosity grades. The HTHS test complements the 100°C viscosity test, specifically measuring the oil film strength during the most demanding moments of the engine, when protection is most needed.
Excessively low HTHS viscosity can result in insufficient lubrication and increased wear.
- Cold Crank Simulator (CCS) viscosity - measures the cold start performance of an engine at low temperatures.
CCS viscosity is a key parameter in defining the "W" (winter) grade. This simulates the resistance to oil flow during engine startup at extremely low temperatures. The lower the value, the less strain on the battery and engine during a cold start, resulting in a smoother start.
- Micro Rotational Viscometer (MRV) / Pumping Viscosity - Can the oil be pumped at low temperatures?
MRV viscosity is another key parameter in defining "W" grade engine oil.
It simulates whether the oil can be picked up by the oil pump and circulated effectively to all engine components at low temperatures. If the MRV is too high, pumping becomes difficult, potentially leading to oil starvation and severe engine wear immediately after starting.
- 40°C Kinematic Viscosity - Used to Calculate Viscosity Index
Although 40°C kinematic viscosity is not directly used in SAE J300 ratings, it remains a very important physical property. It is crucial in oil formulation development and quality control. 40°C kinematic viscosity is primarily used to calculate the viscosity index.
Viscosity Change and Viscosity Index Improvers - SSI
Lubricating oil containing Viscosity Index Improvers (VIIs) shows significantly improved viscosity and a higher viscosity index. However, after a certain period of engine operation, its viscosity may drop noticeably. Why does this happen?
Viscosity Index Improvers are essentially oil-soluble polymers. When the lube oil is subjected to extreme shears-such as in high-speed gears and oil pumps-these polymer chains can be mechanically cut off. This process permanently reduces the molecular weight of the polymers, resulting in a significant decrease in the engine oil's viscosity. This phenomenon is professionally referred to as Permanent Shear Stability Loss. This leads to thinning of the oil film, inadequate lubrication, and reduced sealing performance, which in turn accelerates component wear.
Premium lubricants prioritize polymers with exceptionally high shear stability index (SSI) to ensure that viscosity remains within the designed range throughout the entire oil drain interval, providing consistent and lasting protection for the engine.
In the industrial oil, requirements for viscosity vary widely across different applications.
Taking hydraulic oil as an example, Chinese standards define specifications for L-HM (Anti-wear), L-HV (Low-temperature), and L-HS (Ultra-low-temperature) grades. We can see that although their viscosity requirements at 40°C are the same, their requirements for low-temperature viscosity, pour point, and viscosity index are totally different.
| Property | L-HM (Anti-wear) | L-HV (Low Temperature) | L-HS (Ultra-Low Temperature) |
| Kinematic Viscosity at 40°C (mm²/s) | 28.8 ~ 35.2 | 28.8 ~ 35.2 | 28.8 ~ 35.2 |
| Low-Temperature Startability (Temperature for 1500 mm²/s Viscosity) |
Not Specified | ≤ -18°C | ≤ -24°C |
| Viscosity Index VI | ≥ 95 | ≥ 140 | ≥ 150 |
| Low-Temperature Fluidity (Pour Point) |
≤ -15°C | ≤ -33°C | ≤ -45°C |
| Recommended Application | Moderate/Stable Ambient Temperatures | Wide Temperature Range, Cold Regions | Severe Cold, Arctic Conditions |
Anti-wear Hydraulic Oil - L-HM : This basic grade prioritizes viscosity at room temperature (40°C) and 0°C. It has the lowest requirements for viscosity index and low-temperature performance.
Low-temperature Hydraulic Oil - L-HV : Building on L-HM, this grade significantly improves viscosity index and low-temperature performance, making it suitable for cold climates.
Ultra-low-temperature Hydraulic Oil - L-HS : this grade oil further enhances ultra-low-temperature startability and viscosity index beyond L-HV. It is the highest grade, designed for extreme cold climates.
What components in lubricant formulations affect viscosity?
| CF-4 15W-40 Lubricating Oil Formulation Example | ||
| Component | Percentage | Primary Function & Impact on Viscosity |
| Base Oil-150SN | 53.0 | Provides the formulation base and datum viscosity. |
| Base Oil- 500SN | 29.0 | As a heavy base oil component, it increases the datum viscosity. |
| Lube Additive Package | 8.0 | Provides detergency, dispersancy, anti-wear properties, etc., while also contributing a slight thickening effect. |
| Viscosity Index Improver (OCP) | 9.6 | Dramatically increases high-temperature viscosity to ensure the "40" grade is met and improves the Viscosity Index to achieve the "15W" low-temperature performance. |
| Pour Point Depressant | 0.4 | Improves Low-Temperature Fluidity: Optimizes low-temperature viscosity (e.g., CCS), but with a very low impact on high-temperature viscosity. |
| Antifoam Agent | 10 ppm | Suppresses Foam: Essentially does not affect the overall oil viscosity. |
In the above formulation, the viscosity of the finished lubricating oil is primarily influenced by three key aspects.
Base Oils
It is the main component in the lube oil formulation; its viscosity directly determines the datum viscosity of the finished product.
One or two base oils are selected as the base fluid of a lubricant. e.g., It is a common method to use a mixture of 150N and 500N at a certain ratio to blend a 20W-50 engine oil.
Viscosity Index Improver (VII)
VII significantly thickens the oil and improves its viscosity index. It is the key component enabling multi-grade oils.
Other Additives
The Additive Package ( VIIs are not included) generally also contributes a small thickening effect. Furthermore, additives like pour-point depressants ( e.g., PMA PPD) also slightly increase the viscosity.
In order to get a high-quality lubricating oil, the oil blender engineers should calculate precisely according to raw materials' data sheets, and balance the relationship between these three elements.
