Viscosity of Engine Oils

There are several properties of engine oil that affect its performance but the most significant is viscosity. Which is defined as the resistance of any liquid to flow at a certain temperature.

All motor oils have different resistance to flow at a certain temperature.

This characteristic of engine oil is measured and classified by the Society of Automotive Engineers (SAE) and is known as viscosity grades of engine oil.

They have notated using the categorization “XW-XX” where the engine oil is tested for its viscosity at both zero and hundred degrees Celsius.

The cold temperature viscosity is written with ‘W’ showing winter while the hot temperature viscosity is written after ‘-’.

Find more about the key terminologies and concepts about the viscosity of engine oils in the upcoming headings.

Type of Viscosities

There are basically two types of engine oil viscosities:

Monograde and multigrade.

Monograde represents the viscosity of the oil at a single temperature range e.g., for any winter grade it is -10° to -35° C.

These grades were used to describe lubricants manufactured for older vehicles.

For colder regions, we have to use grades with ‘W’ marking e.g., SAE 5W and for hotter regions, we had to use grades without the ‘W’ marking e.g., SAE 20.

On the other hand, multi-grade oils are more common today and are used in modern vehicles. They show viscosity for both cold and hot temperatures.

In other words, the “XW-XX” categorization described earlier in the introduction is known as multi-grade viscosity.

The main benefit of multi-grade oils is that they could be used for both cold and hot temperatures whereas the monograde oils were restricted to a very small temperature range.

Today you will find almost all of the engine’s oils to be multi-graded.

Different Viscosity grades

There are a limited number of mono-grade oils, but we can combined them to make any number of multi-grade oils as per our needs and preferences.

Mono-Grade OilsMulti-Grade Oils
SAE 15WSAE 0W-30
SAE 20WSAE 5W-20
SAE 25WSAE 5W-30
SAE 30WSAE 5W-50
SAE 8SAE 10W-30
SAE 12SAE 10W-40
SAE 16SAE 10W-50
SAE 20SAE 15W-40
SAE 30SAE 15W-50
SAE 40SAE 15W-60
SAE 50SAE 20W-50
SAE 60SAE 80W-90

Side Note: For better understanding you may want to check the following:

Why do Viscosity Numbers matters?

Viscosity Number is the most crucial property of oil as the whole operation of the engine depends upon it. 

It not only dictates the flow characteristics of the oil inside the engine but also how much effective the oil will be against wear and tear.

Other than that, the key performance indicators of the oil like oil consumption, fuel economy, and viscosity index are also directly linked to it.

How to measure engine oil viscosity?

We cannot measure the viscosity of engine oil through the naked eye or by some tricks if you will as it requires scientific apparatus called a viscometer.

It can either be measured by the Capillary Tube Viscometer test method or through the Rotary viscometer test method.

Don’t worry as we don’t have to perform any of these tests to find the viscosity of engine oil because the manufacturers label each and every bottle of engine oil with their respective grades.

Testing which viscosity grade is the best?

To simply put it, go for the lowest possible cold viscosity for best winter performance and the highest possible hot viscosity for best summer performance.

To test this statement, we can take several different grade oils as test subjects and first cool them to a negative temperature to see whether the oil with the lowest cold viscosity flows the best or not.

Similarly, we can heat our engine oils to a certain high temperature to see whether the oil with the highest hot viscosity flows the slowest or not.

Factors influencing the viscosity of engine oil

The factors influencing the viscosity of engine oil are:

  • Oil Composition
  • Temperature
  • Pressure
  • Dissolved gases
  • Contamination
  • Quality and quantity of viscosity improvers/modifiers.

Effects of viscosity grade on engine performance

A significant decrease in the viscosity of the oil can cause:

  • Deformation of oil film
  • Increase in mechanical friction
  • Excessive energy loss inside the engine
  • Heat generation
  • Leakage of oil through venerable parts
  • Higher effects of oil contaminants

In the case of very higher viscosity, the results aren’t positive either.

We may expect the following to occur:

  • Excessive heat triggers oxidation leading to sludge and varnish buildup.
  • Gaseous cavitation in pumps and gears due to improper oil flow.
  • Lubrication Loss
  • Over consumption of energy to overcome oil friction.
  • Dry start
  • Poor cold start

Importance of Oil Viscosity

There should be no doubt about the importance of the viscosity of oil as we have discussed the negative effects of its imbalance.

The engine will perform best when it gets the optimal thickness of the oil according to the temperature.

This way neither the power of the engine will be lost, nor the extra heat will be generated which is linked to failing to oil film, sludge protection, and in worst case scenario the expanded metal parts of the engine could jam the engine.

Which Engine oil viscosity to use?

There are a few methods through which you could select the best grade of oil possible for your engine. These are:

  • Owner’s Manual

There is no need to overburden yourself with the decision of choosing the right grade of engine oil to use as the manufacturers have already mentioned the recommended grades of oil to use in the manual.

These grades were determined after many tests and are considered the most reliable source of determining which oil grade to use.

  • External Temperature

In some unforsaken circumstances when the luxury of an owner’s manual is not available, there is only one method left to decide the grade of your oil and that method is to consider the extremes of your ambient environment.

Every multi-grade oil has both hot and cold temperature extremes, compare it with the data you have gathered of your area and find the one which best serves you.

Impact of using non-recommended viscosity oil grade in the engine

We have already seen the extreme effects of having too little or too high viscosity of oil inside the engine but what if someone went just a few grades away from the recommended ones?

Will there be any consequence?

The answer is yes, but it may not be as severe as having very low or high yet in the long term it will have effects.

A few of these effects are shared below:

  • Early breakdown of oil molecules
  • Decrease in fuel economy
  • Higher exhaust emissions
  • Delayed start in cold weather
  • Reduced energy efficiency
  • Higher operating temperatures than usual.