Are 5w and 0w oils too thin?
Discussion
I read on many forums about 0w and 5w oils being too thin.
0w-40, 5w-40, 10w-40 and 15w-40 are all the same thickness (14 centistokes) 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 plenty 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.
I hope this helps explain a bit.
Cheers
Guy.
0w-40, 5w-40, 10w-40 and 15w-40 are all the same thickness (14 centistokes) 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 plenty 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.
I hope this helps explain a bit.
Cheers
Guy.
let me try and keep this answer simple.......the thinner the oil you run, and the more revs you run.......the more chance you have of experiencing main and big-end bearing cavitation due to excessive crankshaft velocity inside the shell-bearing housing of the main or big end bearings.....
as the oil will be so thin......the crankshaft flaps around that bit more in the extra space.....and the pounding forces of the crankshaft tear away at the bearing surface....commonly known as bearing fatigue.......the bearing delaminates.....the rod and bearing overheat......the rod breaks and punches a hole in the side of the cylinder block......not good.
so dont get clever with thin oils.......just run stuff that you know is safe.
manufacturers have become very clever at specifying oils that are border-line for their applications, all in order to reduce friction, and increase fuel economy at their reted engine speeds, Ford reccomend 5W30 for pretty much all their diesels.....but.......as soon as the engine revs are increased, when using the same oils......dont be surprised if you hit problems......thats the time to change the oil to something a tad thicker.......its not worth the risk.
as the oil will be so thin......the crankshaft flaps around that bit more in the extra space.....and the pounding forces of the crankshaft tear away at the bearing surface....commonly known as bearing fatigue.......the bearing delaminates.....the rod and bearing overheat......the rod breaks and punches a hole in the side of the cylinder block......not good.
so dont get clever with thin oils.......just run stuff that you know is safe.
manufacturers have become very clever at specifying oils that are border-line for their applications, all in order to reduce friction, and increase fuel economy at their reted engine speeds, Ford reccomend 5W30 for pretty much all their diesels.....but.......as soon as the engine revs are increased, when using the same oils......dont be surprised if you hit problems......thats the time to change the oil to something a tad thicker.......its not worth the risk.
Edited by knighty on Thursday 14th June 14:14
FWDRacer said:
Guy AKA OPie...
All very good information (cheers for the post)..
What happens when you put the gearbox in the sump and the long chain molecules get mashed to bits by the geartrain? Recommendations on cold/hot viscosity still stand?
probably just best to go for a decent synthetic in this case - the base oil is naturally more shear stable, but also has a naturally better VII so requires less polymer (which is even more prone to shear)All very good information (cheers for the post)..
What happens when you put the gearbox in the sump and the long chain molecules get mashed to bits by the geartrain? Recommendations on cold/hot viscosity still stand?
skinny said:
oilman was primarily talking about oils with less viscosity at low temps... probably in response to countless people telling others that 0W-40 oils are too thin and will wreck their engine
I know!......I should have also said many manufacturers have gotten their fingers well and truly burnt by running oil that is way too thin......I should know, as I'm working for the powertrain engineering consultancy thats helps trouble-shoot the problems......I have seen countless pictures of main and big end bearings from failed warranty engines......the most easiest fix to impliment in the field is to use a thicker oil......hence, they should have used a thicker oil to start with.also for your information, castor oils are signed off in production engines at 140DegC.....synthetic oils are signed off at 150DegC.....cheap french engines are even worse......and believe me, the production car oil temps get this high......the worst case test is towing a really big caravan up a hill on a hot summers day when the car is fully laiden with big occupants......this is an important sign-off test for all manufacturers.
Edited by knighty on Thursday 14th June 17:18
yes, I'm quite serious......castor oils are quite safe to this temperature.....any higher and it starts to degrade very quickly.......in race engines people often run their oil way too cool......as a guide you shouldnt be afraid to run well over 100DegC....like 110/120......castor and synthetic oils can quite easily handle this......but certainly dont go above 140/150
knighty said:
yes, I'm quite serious......castor oils are quite safe to this temperature.....any higher and it starts to degrade very quickly.......in race engines people often run their oil way too cool......as a guide you shouldnt be afraid to run well over 100DegC....like 110/120......castor and synthetic oils can quite easily handle this......but certainly dont go above 140/150
i heard castor oils, being vegetable based, start to go off above about 100 deg - or maybe that's the classic stuff like castrol R - i guess the new stuff has some sort of antioxidant in it? didn't think they even still made it apart from for the classic market to be honest...
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