Lets clear up a common misconception (oil)
05-26-2005, 05:17 AM
#1
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Lets clear up a common misconception (oil)
I read on many forums about 0w and 5w oils being too thin. I will try to explain it without getting over technical and we'll go from there.
0w-40, 5w-40, 10w-40 and 15w-40 are all the same thickness (14cst) at 100degC.
Centistokes (cst) is the measure of a fluid's resistance to flow (viscosity). It is calculated in terms of the time required for a standard quantity of fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid.
As viscosity varies with temperature, the value is meaningless unless accompanied by the temperature at which it is measured. In the case of oils, viscosity is generally reported in centistokes (cst) and usually measured at 40degC and 100degC.
So, all oils that end in 40 (sae 40) are around 14cst thickness at 100degC.
This applies to all oils that end in the same number, all oils that end in 50 (sae 50) are around 18.5cst at 100degC and all oils that end in 60 (sae 60) are around 24cst at 100degC.
With me so far?
Great!
Now, ALL oils are thicker when cold. Confused? It's true and here is a table to illustrate this.
SAE 40 (straight 40)
Temp degC.........................Viscosity (thickness)
0..........................................2579cst
20..........................................473cst
40..........................................135cst
60..........................................52.2cs t
100........................................ 14cst
120.........................................8.8cst
As you will see, there is penty of viscosity at 0degC, in fact many times more than at 100degC and this is the problem especially in cold weather, can the oil flow quick enough to protect vital engine parts at start up. Not really!
So, given that an sae 40 is 14cst at 100degC which is adequate viscosity to protect the engine, and much thicker when cold, how can a 0w oil be too thin?
Well, it can't is the truth.
The clever part (thanks to synthetics) is that thin base oils can be used so that start up viscosity (on say a 5w-40 at 0degC) is reduced to around 800cst and this obviously gives much better flow than a monograde sae 40 (2579cst as quoted above).
So, how does this happen, well as explained at the beginning, it's all about temperature, yes a thin base oil is still thicker when cold than at 100degC but the clever stuff (due to synthetics again) is that the chemists are able to build these oils out of molecules that do not thin to less than 14cst at 100degC!
What are the parameters for our recommendations?
Well, we always talk about good cold start protection, by this we mean flow so a 5w will flow better than a 10w and so on. This is why we recommend 5w or 10w as the thickest you want to use except in exceptional circumstances. Flow is critical to protect the engine from wear!
We also talk about oil temps, mods and what the car is used for. This is related to the second number xw-(XX) as there may be issues with oil temperatures causing the oil to be too thin and therefore the possibility of metal to metal contact.
This is difficult to explain but, if for example your oil temp does not exceed 120degC at any time then a good "shear stable" sae 40 is perfectly capable of giving protection.
"Shear stability" is important here because if the oil shears it thins and that's not good!
However, if you are seeing temperatures in excess of 120degC due to mods and track use etc then there is a strong argument to using an sae 50 as it will have more viscosity at these excessive temperatures.
There are trade offs here. Thicker oils cause more friction and therefore more heat and they waste power and affect fuel consumption so it's always best to use the thinnest oil (i.e. second number) that you can get away with and still maintain oil pressure.
There is more but this post is too long already so lets keep it to basics.
Cheers
Simon
0w-40, 5w-40, 10w-40 and 15w-40 are all the same thickness (14cst) at 100degC.
Centistokes (cst) is the measure of a fluid's resistance to flow (viscosity). It is calculated in terms of the time required for a standard quantity of fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid.
As viscosity varies with temperature, the value is meaningless unless accompanied by the temperature at which it is measured. In the case of oils, viscosity is generally reported in centistokes (cst) and usually measured at 40degC and 100degC.
So, all oils that end in 40 (sae 40) are around 14cst thickness at 100degC.
This applies to all oils that end in the same number, all oils that end in 50 (sae 50) are around 18.5cst at 100degC and all oils that end in 60 (sae 60) are around 24cst at 100degC.
With me so far?
Great!
Now, ALL oils are thicker when cold. Confused? It's true and here is a table to illustrate this.
SAE 40 (straight 40)
Temp degC.........................Viscosity (thickness)
0..........................................2579cst
20..........................................473cst
40..........................................135cst
60..........................................52.2cs t
100........................................ 14cst
120.........................................8.8cst
As you will see, there is penty of viscosity at 0degC, in fact many times more than at 100degC and this is the problem especially in cold weather, can the oil flow quick enough to protect vital engine parts at start up. Not really!
So, given that an sae 40 is 14cst at 100degC which is adequate viscosity to protect the engine, and much thicker when cold, how can a 0w oil be too thin?
Well, it can't is the truth.
The clever part (thanks to synthetics) is that thin base oils can be used so that start up viscosity (on say a 5w-40 at 0degC) is reduced to around 800cst and this obviously gives much better flow than a monograde sae 40 (2579cst as quoted above).
So, how does this happen, well as explained at the beginning, it's all about temperature, yes a thin base oil is still thicker when cold than at 100degC but the clever stuff (due to synthetics again) is that the chemists are able to build these oils out of molecules that do not thin to less than 14cst at 100degC!
What are the parameters for our recommendations?
Well, we always talk about good cold start protection, by this we mean flow so a 5w will flow better than a 10w and so on. This is why we recommend 5w or 10w as the thickest you want to use except in exceptional circumstances. Flow is critical to protect the engine from wear!
We also talk about oil temps, mods and what the car is used for. This is related to the second number xw-(XX) as there may be issues with oil temperatures causing the oil to be too thin and therefore the possibility of metal to metal contact.
This is difficult to explain but, if for example your oil temp does not exceed 120degC at any time then a good "shear stable" sae 40 is perfectly capable of giving protection.
"Shear stability" is important here because if the oil shears it thins and that's not good!
However, if you are seeing temperatures in excess of 120degC due to mods and track use etc then there is a strong argument to using an sae 50 as it will have more viscosity at these excessive temperatures.
There are trade offs here. Thicker oils cause more friction and therefore more heat and they waste power and affect fuel consumption so it's always best to use the thinnest oil (i.e. second number) that you can get away with and still maintain oil pressure.
There is more but this post is too long already so lets keep it to basics.
Cheers
Simon
05-26-2005, 05:28 AM
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That's a great post, thank you for taking the time!
Can you explain how the two numbers are measured and calculated? i.e. What does that first number really mean and what does the second number really mean?
Is there a way of measuring or estimating at what temperature a given oil will become useless in an engine?
Thanks again
Can you explain how the two numbers are measured and calculated? i.e. What does that first number really mean and what does the second number really mean?
Is there a way of measuring or estimating at what temperature a given oil will become useless in an engine?
Thanks again
05-26-2005, 05:45 AM
#3
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Thread Starter
In a multigrade, the first number relates to cold crank viscosity and the second is the oil's viscosity throughout the range. It's all about thinning and the oil being prevented from thinning below a certain point at a certain temperature.
An sae 40 (second number) must not thin at 100degC to lower than 14cst otherwise it's not an sae 40.
A normal monograde sae 40 (base oil) will be around 2500cst at 0degC and thins to 14cst at 100degC.
A multigrade, using thinner base oils (say 5w-40) will only be around 800cst at 0degC and again 14cst at 100degC. What this means is they're all the same viscosity at 100degC but the multigrade, mainly due to the use of synthetics in the case of 5w has much better flow at cold start which prevents wear.
All oils shear with use and the life id determined mainly by quality, it's not the basestock but the VI Improvers that shear!
Viscosity Index Improvers.
An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.
VI Improvers are heat sensitive long chain, high molecular weight polymers that increase the relative viscosity of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.
"Shearing"
The long chain molecules in VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing viscosity. This "shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules. The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.
This shearing not only reduces the viscosity of the oil but can cause piston ring sticking (due to deposits), increased oil consumption and increased engine wear.
Like basestock quality, VI Improvers also vary in quality. The best quality ones are normally found in synthetic oils (Group IV - PAO / Group V - Esters) and it is important to understand that the less of these in the oil the better the oil will stay in grade.
Which oils require more VI Improvers?
There are two scenarios where large amounts of these polymers are required as a rule.
Firstly in "wide viscosity" multigrades. By this I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 (diff 45) and 10w-60 (diff 50) are what is termed as "wide viscosity" oils.
Narrow viscosity oils like 0w-30 (diff 30) or 5w-40 (diff 35) require far less VI Improvers and therefore are less prone to "shearing".
Secondly, mineral and hydrocracked (petroleum synthetic oils) require more VI Improvers than proper PAO/Ester (Group IV or V) synthetic oils as they are less thermally stable to begin with and this is due to the non-uniform molecules in petroleum oils as opposed to the uniformity of synthetics built in laboratories by chemists.
It is a fact that some synthetics require little or no VI Improvers to work as a multigrade due to their superior thermal stability.
How to identify a good "shear stable" oil.
API and ACEA both conduct tests called HTHS (High Temperature/ High Shear) and all oils carrying these specifications are tested and scored.
For all oils, these test results are available however, they are often ommitted from the oils technical data sheet! Oil Companies have a tendency to publish the figures that they want you to see and therefore you often need to dig further or ask for certain information when comparing the performance of various oils.
High-Temperature/High-Shear
This test is a simulation of the shearing effects that would occur within an engine. In fact, it's actually designed to simulate motor oil viscosity in operating crankshaft bearings.
Under high stress conditions where shearing can occur, the VI Improvers (polymers) break down. As they do, the viscosity of the oil decreases. This is what the High Temperature/High Shear test checks for.
The HT/HS test is measured in Centipoise (cP) as the Cold Crank Simulator test is. However, in this case, because you're hoping for the least loss of viscosity with an increase in heat and stress, you want the cP value to remain high.
Each SAE multi-viscosity grade has a specific lower limit for the HT/HS cP value. If a multi-viscosity oil cannot achieve a cP value above that limit, it cannot be classified under that viscosity grade. For instance, according to the SAE specifications, an oil must achieve an HT/HS cP value of 3.7 or higher in order to be classified at the 15w40 viscosity grade.
The thinner the oil the lower the number.
Hope this helps
Cheers
Simon
An sae 40 (second number) must not thin at 100degC to lower than 14cst otherwise it's not an sae 40.
A normal monograde sae 40 (base oil) will be around 2500cst at 0degC and thins to 14cst at 100degC.
A multigrade, using thinner base oils (say 5w-40) will only be around 800cst at 0degC and again 14cst at 100degC. What this means is they're all the same viscosity at 100degC but the multigrade, mainly due to the use of synthetics in the case of 5w has much better flow at cold start which prevents wear.
All oils shear with use and the life id determined mainly by quality, it's not the basestock but the VI Improvers that shear!
Viscosity Index Improvers.
An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.
VI Improvers are heat sensitive long chain, high molecular weight polymers that increase the relative viscosity of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.
"Shearing"
The long chain molecules in VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing viscosity. This "shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules. The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.
This shearing not only reduces the viscosity of the oil but can cause piston ring sticking (due to deposits), increased oil consumption and increased engine wear.
Like basestock quality, VI Improvers also vary in quality. The best quality ones are normally found in synthetic oils (Group IV - PAO / Group V - Esters) and it is important to understand that the less of these in the oil the better the oil will stay in grade.
Which oils require more VI Improvers?
There are two scenarios where large amounts of these polymers are required as a rule.
Firstly in "wide viscosity" multigrades. By this I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 (diff 45) and 10w-60 (diff 50) are what is termed as "wide viscosity" oils.
Narrow viscosity oils like 0w-30 (diff 30) or 5w-40 (diff 35) require far less VI Improvers and therefore are less prone to "shearing".
Secondly, mineral and hydrocracked (petroleum synthetic oils) require more VI Improvers than proper PAO/Ester (Group IV or V) synthetic oils as they are less thermally stable to begin with and this is due to the non-uniform molecules in petroleum oils as opposed to the uniformity of synthetics built in laboratories by chemists.
It is a fact that some synthetics require little or no VI Improvers to work as a multigrade due to their superior thermal stability.
How to identify a good "shear stable" oil.
API and ACEA both conduct tests called HTHS (High Temperature/ High Shear) and all oils carrying these specifications are tested and scored.
For all oils, these test results are available however, they are often ommitted from the oils technical data sheet! Oil Companies have a tendency to publish the figures that they want you to see and therefore you often need to dig further or ask for certain information when comparing the performance of various oils.
High-Temperature/High-Shear
This test is a simulation of the shearing effects that would occur within an engine. In fact, it's actually designed to simulate motor oil viscosity in operating crankshaft bearings.
Under high stress conditions where shearing can occur, the VI Improvers (polymers) break down. As they do, the viscosity of the oil decreases. This is what the High Temperature/High Shear test checks for.
The HT/HS test is measured in Centipoise (cP) as the Cold Crank Simulator test is. However, in this case, because you're hoping for the least loss of viscosity with an increase in heat and stress, you want the cP value to remain high.
Each SAE multi-viscosity grade has a specific lower limit for the HT/HS cP value. If a multi-viscosity oil cannot achieve a cP value above that limit, it cannot be classified under that viscosity grade. For instance, according to the SAE specifications, an oil must achieve an HT/HS cP value of 3.7 or higher in order to be classified at the 15w40 viscosity grade.
The thinner the oil the lower the number.
Hope this helps
Cheers
Simon
05-26-2005, 06:10 AM
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Another question for you Simon 
Why is the Silkolene Pro S 5w-40 better than Brand X 5w-40 from Halfords?
I'd like to know as I've just bought 20L of the Silkolene from you so there ought to be a reason!
Why is the Silkolene Pro S 5w-40 better than Brand X 5w-40 from Halfords?
I'd like to know as I've just bought 20L of the Silkolene from you so there ought to be a reason!
05-26-2005, 06:27 AM
#6
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Thread Starter
Originally Posted by tonytjt,May 26 2005, 02:04 PM
This is all very well but I am still none the wiser as to which oil to use in the S2000
so if you can enlighten me I would very much appreciate it.
Tony
so if you can enlighten me I would very much appreciate it.
Tony
Cheers
Simon
05-26-2005, 06:30 AM
#7
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Thread Starter
Originally Posted by GSi,May 26 2005, 02:10 PM
Another question for you Simon
Why is the Silkolene Pro S 5w-40 better than Brand X 5w-40 from Halfords?
I'd like to know as I've just bought 20L of the Silkolene from you so there ought to be a reason!
Why is the Silkolene Pro S 5w-40 better than Brand X 5w-40 from Halfords?
I'd like to know as I've just bought 20L of the Silkolene from you so there ought to be a reason!
This should explain.
Basestock categories and descriptions
All oils are comprised of basestocks and additives. Basestocks make up the majority of the finished product and represent between 75-95%.
Not all basestocks are derived from petroleum, in fact the better quality ones are synthetics made in laboratories by chemists specifically designed for the application for which they are intended.
Basestocks are classified in 5 Groups as follows:
Group I
These are derived from petroleum and are the least refined. These are used in a small amount of automotive oils where the applications are not demanding.
Group II
These are derived from petroleum and are mainly used in mineral automotive oils. Their performance is acceptable with regards to wear, thermal stability and oxidation stability but not so good at lower temperatures.
Group III
These are derived from petroleum but are the most refined of the mineral oil basestocks. They are not chemically engineered like synthetics but offer the highest level of performance of all the petroleum basestocks. They are also known as
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05-26-2005, 06:43 AM
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This is a great post and clears up a lot of the misunderstandings about oil IMO. Thanks very much Simon.
Only 1 question: If you were using your car for occasional track days etc with some minor modifications (air filter & exhaust) what would you recommend? I'm guessing you would want the sae to be higher?
Thanks,
Karl
Only 1 question: If you were using your car for occasional track days etc with some minor modifications (air filter & exhaust) what would you recommend? I'm guessing you would want the sae to be higher?
Thanks,
Karl