# 40 c or 100 c

#### Bill_W

I have seen the explanation here that oils are rated at the 100 c or operating temperature of the engine. This video at 5:30 says the oil is rated at the 40 c temperature.

Not to argue with minds with more knowledge than mine on oil but this makes more sense. The 40 c would need a matching cSt with the base oil VI and/or the VII to get the final operating temperature 100 c rating.

#### kschachn

I thought this was already discussed?

It is standard for the temperature to be either 40 degrees C or 100 degrees C, and there are reasons for measuring viscosity at each of these temperatures.

Besides normalizing temperature for comparative reasons, it's also best to measure an oil's viscosity as close to what the operating temperature would be.

Typically, 40 degrees C is not far from the average industrial machine operating temperature. Also, viscosity changes are more prominent at lower temperatures. Therefore, detecting any abnormalities, such as those influenced by water, fuel or oxidizing oil, would be easier at this lower temperature.

However, many machines operate at higher temperatures, such as diesel or gasoline engines. Thus, it is common for engine oil (and other lubricants under similar conditions) to be tested at 100 degrees C.

#### Bill_W

Thread starter
I thought this was already discussed?

Thanks for the links… But it is easy to get confused when talking about motor oils. As the video points out 40 c is the starting point and most likely the cSt base oil is targeting that number. But some use the 100 c as the ultimate grade in the engine. I do not mind showing my lack of knowledge and not a pro at following past discussions.

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#### RDY4WAR

The kinematic viscosity at 100°C (KV100) is the only one that matters for SAE J300. The KV40 is measured to determine the viscosity index, but not much beyond that. The reason for this is because engines typically operate with oil temperatures closer to 100°C. The exception would be for oils particular drag racing classes where the oil temp is barely above ambient temperature when they roll into the burnout box. In such a case, the KV40 measurement gets closer attention.

#### Bill_W

Thread starter
The kinematic viscosity at 100°C (KV100) is the only one that matters for SAE J300. The KV40 is measured to determine the viscosity index, but not much beyond that. The reason for this is because engines typically operate with oil temperatures closer to 100°C. The exception would be for oils particular drag racing classes where the oil temp is barely above ambient temperature when they roll into the burnout box. In such a case, the KV40 measurement gets closer attention.
When you say it factors into the VI of the oil does that mean where the base oil viscosity begins at 40 c? Seeing that the VII is not a factor at 40 c. or at least that is where the cross point happens? From this video when you talk about 4 cSt base oil you are really talking about 22 cSt at 40 c with the conversion. But the starting point is actually at 40 c to get to the 100 c with the base oil VI and finished oil VII for the running viscosity. Just the way I see it but just my view.

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#### OVERKILL

##### \$100 Site Donor 2021
When you say it factors into the VI of the oil does that mean where the base oil viscosity begins at 40 c? Seeing that the VII is not a factor at 40 c. or at least that is where the cross point happens? From this video when you talk about 4 cSt base oil you are really talking about 22 cSt at 40 c with the conversion. But the starting point is actually at 40 c to get to the 100 c with the base oil VI and finished oil VII for the running viscosity. Just the way I see it but just my view.

You are confusing and conflating things here:

VII - Viscosity Index Improvers, these are your polymers (plastic) slinky chains that expand with heat, and contract with cold, to reduce the rate of change inherent to the oil's natural viscometric characteristics. Remember, ALL oils thicken as they cool, and thin as they heat. VI Improvers affect the rate of change of that curve, preventing the oil from thinning to what it would otherwise be at that temperature, and contracting as the oil cools, which prevents the oil from getting as thick at a lower temperature that it would otherwise be, relative to its hot viscosity.

VI - Viscosity Index: This is a calculated value based on viscosity measured at 40C and 100C. This is the natural rate of change of the base oil's tendency to thicken as it cools and thin as it heats. A higher VI, the less the oil thickens as it cools, and the less it thins as it heats. PAO base oils have VI's around 140 for example, while we regularly see finished products with VI's approaching 200, which is the result of the impact of VII polymers on that tendency.

Viscosity Calculators, like the Widman one, use the 40C and 100C numbers, the same ones we use to calculate VI, to plot viscosity with temperature. We know that these stop being accurate below 0C. This is because wax crystal formation in bases that aren't PAO, start having an impact on viscosity, driving it up. While we can mitigate this tendency somewhat, and push it down further using PPD's (Pour Point Depressants) ultimately, wax crystals will form at some point and the oil will begin to gel and eventually won't be able to pump (if it isn't PAO).

J300:

As @RDY4WAR noted, 40C visc does not show up anywhere on here. The grades are all defined at 100C.

A 4cSt base is the bottom end of the SAE 8 grade. A 4cSt base is used because it has good cold temperature performance, but sacrifices volatility and will of course require more VII to hit our 100C hot target, say 9.3-12.5cSt if we are shooting for a 0w-30.

Base oils are sold and labelled by their 100C visc. Mobil's SpectraSyn 6 for example, is a 6cSt nominal PAO base (at 100C).

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#### Bill_W

Thread starter
While base oil is labeled at 100 c my question is the actual temperature that the base oil is tested.

It says the viscosity is tested at 40 c… but the viscosity index is a measure between two temperatures. So… is the actual viscosity tested at 40 c and then calculated to 100 c and then when tested at 100 c you can determine the VI between the calculated and the real viscosity at 100 c. What number would they use for the base oil 100 c? real or calculated if the standard is 40 c.

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#### OVERKILL

##### \$100 Site Donor 2021
While base oil is labeled at 100 c my question is the actual temperature that the base oil is tested.

????

Base oils list viscosity at different temperatures because this helps blenders determine if they are appropriate for the intended use. Ergo, they are tested at those temperatures, as that's necessary to determine the performance.

That article is NOT about engine oils. And they are talking about testing to confirm that the visc of the product you purchased, matches what it is supposed to be.
These industrial oils are NOT classified via J300, but rather an ISO grade, which is indeed graded per the 40C viscosity:

This does NOT apply to engine oils!

#### ZeeOSix

##### \$100 site donor 2022
While base oil is labeled at 100 c my question is the actual temperature that the base oil is tested.

It says the viscosity is tested at 40 c… but the viscosity index is a measure between two temperatures. So… is the actual viscosity tested at 40 c and then calculated to 100 c and then when tested at 100 c you can determine the VI between the calculated and the real viscosity at 100 c. What number would they use for the base oil 100 c? real or calculated if the standard is 40 c.
The VI is calculated from the KV40 and the KV100. Both KV40 and KV100 are measured ... one is not calculated from the other.

Look at the section "Viscosity Index". There is a formula to calculate it. You can find lots more with some Google action.

#### Bill_W

Thread starter
Both are measured, got that, but from what I read and here on the video viscosity is measured at 40 c and that is the standard viscosity. The 100 c is measured and compared from the theoretical line for that standard viscosity to the actual viscosity at 100 c tested. The difference is the VI of the oil.

#### ZeeOSix

##### \$100 site donor 2022
^^^ Note what it said in the Machinery Lubrication article I posted above about the VI calculation. Two reference oils are used in the calculation.

"It is based on comparing the kinematic viscosity of the test oil at 40°C, with the kinematic viscosity of two reference oils - one of which has a VI of 0, the other with a VI of 100 (Figure 3) - each having the same viscosity at 100ºC as the test oil."

It looks like only the KV40 is used in the calculation formula (shown in the upper RH corner of Fig 3 above), but all 3 oils (the oil being tested, and the two reference oils) must all have the same KV100 ... so KV100 also needs to be measured.

#### Bill_W

Thread starter
^^^ Note what it said in the Machinery Lubrication article I posted above about the VI calculation. Two reference oils are used in the calculation.

"It is based on comparing the kinematic viscosity of the test oil at 40°C, with the kinematic viscosity of two reference oils - one of which has a VI of 0, the other with a VI of 100 (Figure 3) - each having the same viscosity at 100ºC as the test oil."
That shows a cSt of 72 at 40 c of the oil to be considered. Notice what they are measuring at what temperature.

#### ZeeOSix

##### \$100 site donor 2022
That shows a cSt of 72 of the oil to be considered.
Yep ... so it's used in the VI formula to determine the VI of the "considered oil". In this case, the considered oil has a VI of 72, which makes sense because it has to be someplace between 0 and 100 in the example shown in Fig 3.

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#### ZeeOSix

##### \$100 site donor 2022
The formula below is from Ref 4 in the Wikipedia link, which is the SAE test standard [ASTM D2270-10 (2016)] for VI.

NOTE - if the VI of the tested oil is over 100, then the measured KV100 of the tested oil is used in the VI calculation formula. If the VI of the tested oil is less than or equal to 100, then it would be calculated like the example shown in Fig 3 above.

Source:

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