Viscosity Explained

In this article...

  • What is Viscosity? 
  • How is viscosity rated on engine oils? 
  • What is the importance of viscosity in my engine? 
  • What defines an oil’s Viscosity? 
  • How does Temperature affect Viscosity? 
  • What is Viscosity Index (VI)? 
  • What is a multi-grade oil?


What is Viscosity?

Viscosity, by definition, is an oil’s resistance to flow and shear.  It is the single most critical physical property of the oil as it affects both the wear rate and the fuel efficiency.

Water is comparatively a low viscosity fluid; syrup is a comparatively high viscosity fluid.  With oil, like syrup, as you increase the temperature, the viscosity lowers, meaning it flows faster, or more easily.

The most common unit of measure for viscosity is the Kinematic Viscosity and this is quoted in data sheets at 40°C and 100°C.  The commonly used unit of measure is centistokes but the correct SI unit of measure is mm2/s. 

Kinematic Viscosity is a measure of the fluids resistance to flow and shear under the forces of gravity, or how easily the oil flows to the different parts of the engine.

Kinematic Viscosity (ASTM D445/ISO3105): 1 centi-Stoke (cSt) = 1 mm2/s.

Absolute Viscosity is a measure of a fluid's internal resistance to flow and may be thought of as a measure of fluid friction and of the oil's film strength to support a load.

Dynamic or Absolute Viscosity: 1 milliPascal second (mPa·s) = 1 centi-Poise (cP)

High-temperature high-shear-rate (HTHS) viscosity is an indicator of an engine oil's resistance to flow in the narrow confines between fast moving parts in fully warmed  up engines. The most common test for this is ASTM D 4683 as it closely mimics the conditions found in an engine's crankshaft and connecting rod journal bearings, as well as other narrow regions such as between the cam and follower on flat bucket tappets. This measurement influences such factors as fuel consumption, valve-train wear and bearing protection.

Cold cranking viscosity simulates the viscosity of an oil in crankshaft bearings during start up on a cold winter morning. The test determines if an engine can be cranked over fast enough to start under extreme cold ambient conditions. ASTM Method D 5293 simulates an oil's cranking resistance when cold, and thus indicates the lowest temperature at which an engine is likely to start.

Cold pumpability measures the resistance of an oil to pumping through the engine after a cold start. The most widely used test is ASTM D 4684. If an oil's viscosity becomes too high, pumping will be hindered with possible cavitation issues. Viscosity here becomes an important factor in determining whether the engine runs with sufficient lubrication after starting in severe cold conditions.  The Cold Pumpability test is always conducted at 5°C colder than the Cold Cranking test to ensure the pump can deliver the oil to the bearings.

How is viscosity rated on engine oils?

The Society of Automotive Engineers (SAE) developed a scale for both engine and transmission oils.  The measurement is undertaken in a laboratory in accordance with standard procedures.  W is Winter and oils with the W must meet the requirements of the Cold Cranking and Cold Pumping criteria.

Table 3 – SAE Viscosity Grades - Engines

SAE Viscosity Grades for Engine Oils (SAE J300)  - December 1999


Absolute Viscosity (cP)

Kinematic Viscosity (cSt)

HTHS (cP) 150°C

Maximum Cold Cranking

Maximum Cold Pumping



High-Temperature- High-Shear


6 200 @ -35°C

60 000 @ -40°C





6 600 @ -30°C

60 000 @ -35°C





7 000 @ -25°C

60 000 @ -30°C





7 000 @ -20°C

60 000 @ -25°C





9 500 @ -15°C

60 000 @ -20°C





13 000 @ -10°C

60 000 @ -15°C





















2.9  (1)






3.7  (2)














(1) - for 0W-40, 5W-40 & 10W-40 oils

(2) - for 15W-40, 20W-40, 25W-40 and SAE40 oils

Table 4 – SAE Viscosity Grades - Transmissions

J306 Viscosity Classification  for Automotive Gear Oils

Effective from 1st January 2005

SAE Viscosity Grade

Maximum Temperature for a viscosity of 150,000 cP (°C)

Minimum Viscosity at (cSt) a 100°C

Maximum Viscosity at (cSt) a 100°C


ASTM D 2983

ASTM D 445

ASTM D 445















































Must maintain its viscosity after 20 hours in the CEC L-45-A-99 test.


Table 5 – SAE Viscosity Grades Compared to each other


What is the importance of viscosity in my engine?

The viscosity will determine how easily the oil is pumped to the working components, how easily it will pass through the filter, and how quickly it will drain back to the engine.  The lower the viscosity the easier all this will happen.  That is why cold starts are so critical to an engine because the oil is cold, and so relatively thick.

But, the lower the viscosity, the less the load the oil can support at the bearing on the crankshaft.  The higher the viscosity, the better the load it can support.  Even this, however, has a trade-off, since the higher the viscosity, the more the drag at the bearing, and hence, potential power loss, or increased fuel consumption.  So a compromise is chosen to minimise power loss, but maximise load support. 

For domestic use, engine life is important, and in the main you should adhere to the recommended viscosity for your engine.  For motorsport, engine life is not critical, winning is, so these high performance engines can use lower viscosity oils to maximise power output to the wheels, but then again they generate a lot more heat so may use a higher viscosity anyway.

What defines an oil’s Viscosity?

The oil’s viscosity is defined in the main by the size of the molecules.  The larger the molecule structures, the thicker, or higher, or heavier the viscosity.

The size and structure of mineral oil molecules vary, so the average molecule size dictates the viscosity, whereas the synthetic oil manufacturing process results in consistently sized molecules of an identical structure.

As mineral oil ages, the molecular size and structure changes and hence, the resulting changes in viscosity over the service life of the oil. 


Figure 4 – Mineral vs. Synthetic Oil – Molecule size.

How does Temperature affect Viscosity?

Graph 1 – Viscosity and Temperature relationship for an oil

The viscosity of an oil changes - dependant on the temperature, the pressure, and the actual shear stress on the oil as a result of flowing between moving components.  The viscosity of the engine oil will depend on whether the oil is, for example, in the sump, the pump, the cooler, between the cam and followers, or in the crankshaft bearings.

 The rate at which viscosity changes with respect to temperature is non-linear.  To put this in to perspective, Graph 1 above is commonly used to ascertain the viscosity of a fluid at varying temperature points based on the two known viscosity values quoted at 40°C and 100°C.  In order to draw a straight line, the viscosity is plotted as logarithmic function against the linear temperature scale.

To put this into perspective, this means that for a 10°C increase at low temperature, the viscosity may thin out by more than 500cSt.  However, the same 10°C difference at normal running temperature will result in a change of less than 5cSt.

The ideal viscosity at a bearing is around 10cSt or higher depending on various factors such as the load, rpm and design or dimensions of the bearing shells.  Much higher than this and drag results, much lower than this and boundary lubrication occurs, so you can see why it is critical to keep the engine at an optimum temperature point during running so it is not too hot in Summer or too cold in Winter.  And add in the cold starting problem in Winter and you can see why Viscosity Index is a crucial factor in choosing the right oil.

What is Viscosity Index (VI)?

It is the rate of change of viscosity between two temperatures.  The lower the VI, the more the drop in viscosity as the oil warms up.  The higher the VI value, the less the drop in viscosity as the oil warms up. 

As stated above, the oil’s viscosity will thin out rapidly at first when cold, the rate of thinning then slowing as the engine warms up and will stabilise as running temperature is reached.  It is during the cold start where the concern is greatest owing to the heavy drag from the treacle like oil.  It can take up to 90seconds for the oil to properly circulate in an engine following a dry start from cold.

Ideally, oil would have flat viscosity line with no change in viscosity irrespective of temperature.


Graph 2 – Comparing the VI of two different SAE 30 oils.

But as this ideal oil does not exist, then the Viscosity Index can be improved to reduce the impact.  Generally this is modified by the inclusion of additives known as VI Improvers to form multi-grade viscosity oils.  Synthetics usually have a naturally higher viscosity index than even the artificially increased VI of mineral oils.   Group 2 & 3 oils also have a higher VI than cheap Group 1 oils.  See Base Oils Explained

What this means is that in the example in Graph 2 the SAE30 – Oil B has a lower VI than SAE 30 – Oil A.  There are two benefits to a higher VI oil.  Not only is the oil thinner when cold, but it is thicker when warm.  A higher VI oil can only be of benefit to a car owner living in a cold climate in Winter months. 

What is a multi-grade oil?

Up until the 1960s it was quite common to change the oil depending on the season and old handbooks from the period recommend oils as low as SAE 5W for very cold ambient conditions.  SAE 30 and 40 grades would be typical for Summer use, and sometimes a SAE 50 grade would be substituted if the engine started to use more oil. Multi-grade oils were developed to allow year round use as a result of artificially raising the VI of the oil.  

Scientists discovered the use of polymers and the fact that these would expand with heat.  VI Improvers are polymer material that expands as the oil warms up.  This does NOT thicken the oil as is often stated.  It merely slows down the rate at which oil thins out as the temperature rises.  The base oil in use in a multi-grade (aka multi-seasonal) is the first number (such as 10W or 20W), where as the second grade is the viscosity achieved using the VI Improver (such as 40 or 50).  20W50 is a base oil of SAE 20W and the viscosity grade at running temperature is SAE 50.


Graph 3 – A Multi-Grade/Multi-Seasonal oil operates within the safe limits of Too Thick and Too Thin.

However, these VI Improver additives are susceptible to shearing damage from the mechanical components and would lose some of their performance, resulting in 20W50 oil acting more like 15W40 oil.  This damage can happen very early in the life of the oil and is seen as part of the bedding in of the oil following an oil change.   This is why the oil pressure often appears to have improved just after a fresh oil change as the oil is yet to bed in so is consequently thicker giving a false impression of improved oil pressure.

Historically, with older engines having looser clearances from wear, and especially in the Mini/1100/Metro’s (with the gearbox in the sump resulting in rampant shear damage to the VI Improvers), these engines still generally require a thicker oil such as a mineral 20W50 or 15W40.  In fact it was the launch of the Mini in 1959 that resulted in the 20W50 scenario with BMC/BL products.  The shearing damage on the typical 15W40 caused problems and I am told by reliable sources that the resulting 20W50 was selected to counter this shearing action.  20W50 then seemingly became the defacto oil choice for BL products in the 1970s and is trotted out as the required specification on every forum irrespective of the ambient temperatures.  

In the case of the B Series engine, it must be remembered that the early units used in the MGB between 1962 and 1965 were also a 3 main bearing design and owing to bearing failure I am told the required viscosity was increased.  In addition a cooler was added as standard yet this is often quoted by MG mechanics as being unnecessary and that a thermostat is recommended.  In this case, with a 5 main bearing engine and a cooler without a thermostat one must wonder if a 20W50 is right for these engines or not?  A high VI 10W40 or 0W40 would surely suffice?

20W50 is still used in more modern cars only where extreme ambient heat is encountered for some of the off-road 4x4 pick-ups, such as in Western Australia and the Middle East. 

Modern cars are now designed to run on 10W30 or even 0W30.  This is partly due to tighter clearances in the machining processes now used, but also to reduce fuel consumption even further and meet the ever tightening Euro specifications on emissions.  Owing to the use of synthetics now, the 0W30 oils do not necessarily result in more wear as the film strength of synthetics is superior to mineral oils.

Historically, the US have long used 10W30 for other reasons, in part, I was told, this being to prevent bore polishing as a result of the ash deposits that occurred on US design engines with a greater clearance between the piston top and the cylinder wall.

Guide to SAE Viscosity Grades of Engine Oils for Passenger Cars

Multigrade oils such as SAE 5W-30 and 10W-30 are widely used because, under all but extremely hot or cold conditions, they are thin enough to flow at low temperatures and thick enough to perform satisfactorily at high temperatures.  Note that vehicle requirements may vary. Follow your vehicle manufacturer’s recommendations on SAE oil viscosity grade.

If the lowest outdoor expected temperature is

Typical SAE Viscosity Grades for Passenger Cars

0°C (32°F)

5W-20, 5W-30, 10W-40, 20W-50

-18°C (0°F)

5W-20, 5W-30, 10W-40

Below -18°C (0°F)

5W-20, 5W-30

 Table 5 – SAE recommended Viscosity Grades

The above SAE table describes the typically recommended viscosity grades based on ambient temperatures, leading people, in my opinion, to think that 0W40 oil is overkill for British conditions.  As noted, Synthetics usually have a naturally higher viscosity index than even the artificially increased VI of mineral oils.  Therefore, a 0W40 is better than a 10W40, since both offer the same viscosity (SAE40) at running temperature, but the SAE 0W will circulate more readily during a cold start than a SAE 10W.  It is patently incorrect, as is often stated, that 0W40 is too thin when compared to a 10W40.  How is it too thin?  In fact the performance of a cheap 20W50 sheared down in service is likely to be far worse than that of a 0W40 at running temperatures.  See Graph 4 below.  

Graph 4 – Comparing Viscosity Grades during Warm-up