Oil - The importance of shear stability
Discussion
How to make a complicated subject uncomplicated is not easy so I'll try to explain in the best way I can why oils degrade (shear) with use and the easiest way to identify an oil that will "stay in grade" for longer.
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.
Comparisons of HTHS numbers.
Here for comparison sake are a few numbers that we have compiled from data sheets and requests to the oil companies concerned. These are well known oils and considered to be "quality" synthetics so these comparisons are relevant.
Silkolene PRO S 5w-40
HTHS 4.07
Motul 300V 5w-40
HTHS 4.51
Silkolene PRO S 10w-50
HTHS 5.11
Motul 300V 10w-40
HTHS 4.19
Silkolene PRO R 15w-50
HTHS 5.23
Motul 300V 15w-50
HTHS 5.33
Mobil 1 Motorsport 15w-50
HTHS 5.11
Castrol RS 10w-60
HTHS 3.70
I'm sorry if this is too complicated but making a complicated subject simple is not easy however if I've lost anyone, please feel free to ask questions.
Cheers,
GUY
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.
Comparisons of HTHS numbers.
Here for comparison sake are a few numbers that we have compiled from data sheets and requests to the oil companies concerned. These are well known oils and considered to be "quality" synthetics so these comparisons are relevant.
Silkolene PRO S 5w-40
HTHS 4.07
Motul 300V 5w-40
HTHS 4.51
Silkolene PRO S 10w-50
HTHS 5.11
Motul 300V 10w-40
HTHS 4.19
Silkolene PRO R 15w-50
HTHS 5.23
Motul 300V 15w-50
HTHS 5.33
Mobil 1 Motorsport 15w-50
HTHS 5.11
Castrol RS 10w-60
HTHS 3.70
I'm sorry if this is too complicated but making a complicated subject simple is not easy however if I've lost anyone, please feel free to ask questions.
Cheers,
GUY
MGBV8 said:
Guy
The Castrol 10W60 looked odd so checked with another site and its 5.7, could be a typo on your data sheet as 3.7 would be well out of spec.
Can you give me a link to the site where you found this as I would like to go and have a look, the data sheets I have should be correct, and was up dated October 03.
www.opieoils.co.uk/TechSpecs/CastrolRS10w-60.pdf
I do have an idea on what it could be, but I need more info first.
Cheers
Guy.
>> Edited by opieoilman on Thursday 17th March 09:08
>> Edited by opieoilman on Thursday 17th March 09:10
Guy,
From Italy (Down under 5.4)(German HTHS would also be interesting!)
www.castrol.com/liveassets/bp_internet/castrol/castrol_italy/STAGING/local_assets/downloads/t/TDS_Formula_RS10W60.pdf
This is a fixed temp test
If a thin synthetic runs cooler than a thick mineral comparisons are not really on same basis.
Esters also play a part Redline 5W20 is 3.3 compared to M1 0W40 at 3.6.
Any thoughts on how the basic numbers of vis at 40c 100c and HTHS are effected when oil is used, the effect of shear on vis improvers and shear in relation to aw packages, plus fuel dilution issues for carbed cars rather than FI
From Italy (Down under 5.4)(German HTHS would also be interesting!)
www.castrol.com/liveassets/bp_internet/castrol/castrol_italy/STAGING/local_assets/downloads/t/TDS_Formula_RS10W60.pdf
This is a fixed temp test
If a thin synthetic runs cooler than a thick mineral comparisons are not really on same basis.
Esters also play a part Redline 5W20 is 3.3 compared to M1 0W40 at 3.6.
Any thoughts on how the basic numbers of vis at 40c 100c and HTHS are effected when oil is used, the effect of shear on vis improvers and shear in relation to aw packages, plus fuel dilution issues for carbed cars rather than FI
opieoilman said:
Guy, if you're up this end of Cornwall, pop in for a chat. I have loads of questions about oil which only a good conversation would solve.
Stu
Stu,
Where abouts are you fella? I am based in Redruth but do get around.
Cheers
Guy.
I do loads of surfing and will pop in if I'm down your way in. I'm off just off Whitsand Bay in a little village, St John, pub is absolutely legendary, lots of petrolheads around. If you drive through the village, mine is the house with a hovercraft outside
Whitsand Bay, thats in Devon isnt it?? I will have to get my passport renewed before I can cross the border.
I am a wind surfer myself, I got fed up of paddling all the time, put a sail on and you can use the wave in both directions!!
Next time your in the area, do pop in for a chat.
Cheers
Guy.
>> Edited by opieoilman on Thursday 17th March 17:39
I am a wind surfer myself, I got fed up of paddling all the time, put a sail on and you can use the wave in both directions!!
Next time your in the area, do pop in for a chat.
Cheers
Guy.
>> Edited by opieoilman on Thursday 17th March 17:39
Heres a question for those of you that have good understanding of all things oil:
I have always been puzzled when an oil is said to be too thin when cold, as in the following extract from the FAO Mr Oilman thread:
"The Mobil 1 15w-50 would be fine, the 0w-40 you may find a little thin when cold"
Since oil is thicker when cold, and thinner when hot (in other words at operating temperature), how can a oil that is cold ever be too thin? A cold oil would surely always be thicker than a hot oil; what I mean is that (for instance) a 0W-60 oil is much thicker when cold than when hot. This being the case an oil with the lowest 'W' rating (and therefore thinnest oil) should be sought (I don't know if ratings can go below 0W), and that, in fact, the only really important number is that which refers to the thickness of the oil at operating temperature. If the above is true, I cannot understand why, for instance, a 25W-50 would ever be purchased over a 0W-50, since presumably there is more engine wear when the engine is cold because the oil is too thick.
I hope my question makes sense, and that someone can provide an enlightened opinion.
Regards
I have always been puzzled when an oil is said to be too thin when cold, as in the following extract from the FAO Mr Oilman thread:
"The Mobil 1 15w-50 would be fine, the 0w-40 you may find a little thin when cold"
Since oil is thicker when cold, and thinner when hot (in other words at operating temperature), how can a oil that is cold ever be too thin? A cold oil would surely always be thicker than a hot oil; what I mean is that (for instance) a 0W-60 oil is much thicker when cold than when hot. This being the case an oil with the lowest 'W' rating (and therefore thinnest oil) should be sought (I don't know if ratings can go below 0W), and that, in fact, the only really important number is that which refers to the thickness of the oil at operating temperature. If the above is true, I cannot understand why, for instance, a 25W-50 would ever be purchased over a 0W-50, since presumably there is more engine wear when the engine is cold because the oil is too thick.
I hope my question makes sense, and that someone can provide an enlightened opinion.
Regards
Thanks for the quick reply Guy.
I am still a little confused by the suggestion that a 0W or 5W can cause leaks in older engines; these oils are still thicker than the when the engine is up to operating temperature, therefore if any leak is going to occur it will surely happen when the oil is hot and at it's thinnest.
In a perfect world I would expect an oil that is as thin when cold (at a standard temperature of say 20 degrees celsius) as when hot is the ultimate achievement, something like a -75W-40 (for example) and that it is only the inability to produce chains of molecules that thicken the oil to a sufficient degree when warm that makes this unacheivable. If this were the case engines would have as close to an infinite lifespan as possible.
Regards
I am still a little confused by the suggestion that a 0W or 5W can cause leaks in older engines; these oils are still thicker than the when the engine is up to operating temperature, therefore if any leak is going to occur it will surely happen when the oil is hot and at it's thinnest.
In a perfect world I would expect an oil that is as thin when cold (at a standard temperature of say 20 degrees celsius) as when hot is the ultimate achievement, something like a -75W-40 (for example) and that it is only the inability to produce chains of molecules that thicken the oil to a sufficient degree when warm that makes this unacheivable. If this were the case engines would have as close to an infinite lifespan as possible.
Regards
IMO too thin is a myth in this context. However, synthetics have different chemistry and seals may need to get used to this if run on Mineral for some time. Also mineral creates crud that prevents leaks and a change to Synthetic will clean engine and uncover leaks.
Mineral oils build from a thin base and add vis improvers. Synthetic can use a thick base as naturally flow at lower temperature. Redline is basically a straight weight oil so 40wt is also 15W40.
Before up to operating temperature the oil could also have esters and also anti wear additives Zinc Boron and Moly to help prevent wear.
Mineral oils build from a thin base and add vis improvers. Synthetic can use a thick base as naturally flow at lower temperature. Redline is basically a straight weight oil so 40wt is also 15W40.
Before up to operating temperature the oil could also have esters and also anti wear additives Zinc Boron and Moly to help prevent wear.
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