What are the ISO and SAE viscosity grade classification systems?

29ce0c62 5753 468a ae31 161378d50e9c 1024x646 What are the ISO and SAE viscosity grade classification systems?
The ISO Viscosity Grade system is very simple. The grade is the viscosity at 40’C plus or minus 10%. So if you have an ISO VG 100 that means the viscosity will be between 90 and 110 mm2/s at 40’C to be considered in grade. It’s that simple.

Sometimes you take some things for granted. You get so engrained in industry terms you realise some concepts are not self-explanatory.

I was speaking to a customer the other day and was surprised even though they used ISO VG oils on hundreds of machines at their facility and were talking about switching from one grade to another they still didn’t know what it actually meant.

Hence, this article makes no assumptions of any prior knowledge to explain viscosity classification.

What is viscosity?

Viscosity is the most important property of oil. It is defined officially as “resistance to flow”, but put more simply it is how thick or thin an oil is. The thicker the oil the more separation you get between moving parts in the machine, which means less metal to metal contact, less wear and generally a longer life of the oil.

There are a few classifications of viscosity, but the two main classifications you will come across are ISO and SAE. ISO (international standards organisation) is the European derived system and SAE (society of automotive engineers) is the American derived system. Although both tend to be used internationally.

When you buy lubricating oils (greases have another viscosity classification called NLGI) you will buy them with either an ISO or SAE grade.

Both grading systems will have large differences in potential viscosities with some being almost water thickness all the way up to oils that almost don’t pour at room temperature.

Where are ISO and SAE oils used?

As a general rule industrial gears, compressors and hydraulic machinery will use ISO systems and anything that is an engine or moves (cars, trucks, tractors, vans, trucks, diggers etc) will use SAE. There are a few exceptions and some SAE oils can be used in exchange for an ISO oil and vice versa. You can pretty much be sure that Engines lubricants will always be SAE lubricants though.

Why don’t we all just use one thick oil?

Thicker oils do help reduce wear, but if you imagine your legs are geared teeth and try to walk through a river of thick treacle vs a river of water you will find it would be very hard work through the treacle. This means you would end up being slower, burning more calories and working up a sweat. In machinery terms, thicker oils limits speed, increases energy consumption and can lead to overheating. Hence it is a balance between wear protection and energy efficiency.

Modern lubricants especially in automotive settings tend to favour fuel efficiency much more than lubricant film thickness and hence it is even more important to monitor these systems’ viscosities for potential drops as they are blended to only give the minimum thickness to reduce wear with no wriggle room. Hence fuel dilution can quickly take you beneath these thicknesses and why it is important to monitor the viscosity of your oil.

Is viscosity constant?

In short, No. Viscosity is not the same for oil in all conditions and it changes with temperature. Viscosity is thicker when it’s colder and thinner when it’s hotter. You may have seen this when you put a small spoonful of cooking oil in your pan it may be much thicker when you pour it in, but when it heats up it seems to spread out covering the entire pan surface and seems to move more freely around the pan.

What you are seeing is the viscosity drop with increasing temperature, the film thickness of the oil reducing and resistance to flow (the official definition of viscosity) around the pan reducing.

So when we talk about viscosity it is important to not only say how thick an oil is but the temperature measured because a very thick almost difficult to pour oil in the Antarctic is actually very thin in a nearly 100’C engine.

Hence the two classification systems have specific temperatures to test to determine the viscosity.

What is the ISO VG system?

This is a really simple system. The agreed temperature to test is 40’C.

“An ISO VG grade is the viscosity in mm2/s at 40’C +/- 10%.”

Definition of an ISO VG grade by Learn Oil Analysis . com founder Adam.

Put simply an iso VG 100 is 100mms/2 (mm2/s is also known as cSt in the imperial system naming) +/- 10%. So a value of 90 to 110 would be considered an ISO VG 100 as within 10% of the 100 value. There are a few grades in the system (24), but you only need to remember 6 to work out the rest as I will show you now.

How do I remember all the ISO viscosity grades?

I had a new starter ask me this question. I had to think as I just knew them 24 grades all. Hence I found a way to remember with a few rules and only 6 numbers to remember.

Rule 1

ISO viscosity grades go from 2 to 6800 mms/2 though in practice you won’t see more than

Rule 2

Remember this list of hydraulic grades.

  • 10
  • 15
  • 22
  • 32
  • 46
  • 68

Rule 3

Multiply or divide in multiples of 10 to get all the grades.

The gear oils are just a multiple of 10 of these or a division of these.

So 10 x 10 is 100 etc to make

  • 100
  • 150
  • 220
  • 320
  • 460
  • 680

In practice, these cover 99% of the ISO VG oils on the market. However, for specialist applications, you may need an extra thin or thick oil. So you multiply by 10 again or divide the original list by 10 to get the extra thin oils.

Extra thick oils

  • 1000
  • 1500
  • 2200
  • 3200
  • 4600
  • 6800

Extra thin oils.

  • The lowest grade is 2 so no ISO 1 and 1.5 rounds to 2 so can’t have two iso 2s, so the first two grades are skipped.
  • 22 / 10 = 2.2 which rounded to a whole number is 2
  • 32/ 10 = 3.2 which rounds to 3.
  • 5 (4.6 rounded to a whole number is 5)
  • 7 (6.8 rounded to a whole number is 7)

What is the SAE viscosity system?

The SAE system is a little more complicated and works on a few different viscosity like readings including cold temperature measurements. These are useful for new oil measurements but in used oil measurements these are never performed so in terms of condition monitoring the only useful measurement is the viscosity at 100’C for determining if in the specification or not. SAE grades are the only oils to have the option of multi-grades. These are oils that contain polymer additives that allow the use of a thinner oil that performs better in the cold (W for winter) uncoil when hot to give extra resistance to flow (viscosity), meaning the drop in viscosity with temperature rise is less. You can recognise these oils as they have a W in the name. e.g. a 10W40 is an oil that is much thinner than a pure SAE 40 oil, but at hot temperatures, it acts the same as an SAE 40. Hence when measuring at 100’C we are only determining the last part of the grade. i.e you wouldn’t be able to tell the difference between a 5W40, 10W40, 15W40 and an SAE 40 monograde based on that one measurement. For new oils you would use something called a cold-cranking simulator to determine the definitively before the W part of the oil, but outside of new oil testing, this is not something that is ever performed as a condition monitoring test. Hence we have to rely on other tests such as the viscosity at 40’C to give a good indication as to the temperature at less than 100’C temps. 

It is worth noting for trending you can use the viscosity at 40 as it is more sensitive to viscosity changes related to e.g. high soot or fuel dilution than the viscosity at 100’C. Hence the viscosity at 100’C is more a confirmation of grade test than used always for condition monitoring trending, but some customers like to have it still, in which case you would do both a 40 and 100 reading and a viscosity index measurement.

The grading system does not have a nice methodology like the ISO system and feels like someone used a ruler down a list of measurements to split the results and name them rather than the results having any relation to the measurement.

So for example an SAE 40 is 12.5 to 16.3, SAE30 is 9.3 to 12.5 etc. This is something you can either memorise like me or use a lookup table and I would recommend the latter. Our what grade checker below will help you determine what grade an SAE oil matches by selecting the 100’C measurement and what ISO grade it matches for the 40’C measurement. Have a play and get to learn the grading systems.

What grade am I?

Use our online grade determination tool below to work out what grade your oil might be from the viscosity measurement. It will tell you the ISO VG grade for temperatures at 40’C and the SAE monograde/everything after the W of a multigrade.


can’t see full grades list? Pop out in a new window  

Video summary

Finally, to bring all this together watch a brief training video on viscosity and you should be much more informed as to the test and what it means in future.