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Let's clear up a common misconception!


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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?




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)






60..........................................52.2cs t

100........................................ 14cst



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.






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Sorry I've been away for a while and not searching out the oil threads to reply to but I have been unable to access the forum and busy with preparations for the "show season".


I've no objections to it being in low flying, alongside my ad would be nice!






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Indeed, it merely masks the problem and does nothing to cure it. It simply quietens things down and makes you feel better.


In my opinion using a 5w-40 decent synthetic, containing ester if possible is a better solution than using an sae 50 or sae 60 so long as oil temps and oil pressure are not a problem. It will not cure the rattles but it will mean less engine wear and more BHP.




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Having brisky read this thread you have not drawn risk of aeriation of cavitation into the equation. This is why we tend to use thicker oils, coupled with the fact that although 10cst may protect you engine, it does not produce comforting readings on ones oil pressure gauge when the car is driven very hard.

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Aeration and cavitation will always be more likely with a thick oil.


As for oil pressure, as I've mentioned above thinner oils are better if oil temps are normal and as long as adequate oil pressure can be maintained.


I recently had a discussion with John Rowland of Silkolene concerning oil pressure and his comments may be of interest at this point.


Oil pressure is initially defined by engine design, and it doesn’t have to be high. The mighty Cosworth DFV V-8 which dominated F1 for 15+ years was (and still is) happy with about 45PSI. The Le Mans-winning Bentleys of the 1920s were designed to run on 5-10PSI once the oil was hot.


Obviously, an engine which would always give massive oil pressure would be one built with microscopic bearing clearances, so there would be nowhere for the oil to go! 150PSI, but total seizure in a few seconds!


On the other hand, oil pressure significantly lower than the spec. minimum means that bearings ‘at the end of the queue’ for oil feed get a low flow rate, which is not desirable. Even so, a good flow rate with a wear-resistant low-viscosity oil will improve cooling and be better for the engine than another 10PSI with a thicker oil….and less chance of cavitation problems.




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Sounds like increased pump capacity could be the thing to have. If I have my sums right the power required to pump 20 litres of oil per minute at 5 Bar is only 170 watts (0.23 Hp) so a larger capacity pump is not exactly going to slow you up much. If it safely allows a lower viscosity oil to be used perhaps it would more than pay back its extra power consumption in reduced oil drag in the bearings etc.


Simon - do you have a figure for how much power might be saved from lower viscosity oil?



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These figures may be of interest!


Even More Power!


He place to look for extra power is in that 6% lost as oil drag. In a well-designed modern motor, the oil doesn’t have to cover up for wide clearances, poor oil pump capacity or flexy crankshafts, so it can be quite thin. How thin? Well take a look at these dyno results.


A while ago now, we ran three Silkolene performance oils in a Honda Blackbird motorcycle. this fearsome device is fitted with a light, compact, naturally aspirated 1100cc engine which turns out 120+ bhp at the back wheel. The normal fill for this one-year-old engine was 15w-50, so the first reading was taken using a fresh sump-fill of this grade. (The dyno was set up for EEC horsepower, i.e. Pessimistic)



Max Power 127.9 bhp @ 9750 rpm

Torque 75.8 ft-lbs @ 7300 rpm


After a flush-out and fill up with 5w-40 the readings were;



Max Power 131.6 bhp @ 9750 rpm

Torque 77.7 ft-lbs @ 7400 rpm


Then we tried an experimental grade, 0w-20 yes, 0w-20! This wasn’t as risky as you may think, because this grade had already done a season’s racing with the Kawasaki World Superbike Team, giving them some useful extra power with no reliability problems. (But it must be said, they were only interested in 200 frantic miles before the engines went back to Japan)



Max Power 134.4 bhp @ 9750 rpm

Torque 78.9 ft-lbs @ 7400 rpm


In other words, 3.7 bhp / 2.9% increase from 15w-50 to 5w-40, a 2.8 bhp / 2.1% increase from 5w-40 to 0w-20 or a 6.5 bhp / 5% overall. Not bad, just for changing the oil! More to the point, a keen bike owner would have paid at least £1000 to see less improvement than this using the conventional approach of exhaust/intake mods, ignition re-mapping etc.




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Hi Alex


I would think the gains would apply to a dry sump engine. The windage losses are due to inertial forces (the crank picking up the oil accelerating it and flinging it against the crankcase walls) this is almost entirely a function of the density of the oil and not very dependant on the viscosity. By contrast the losses in sliding close mated surfaces like bearings etc will be almost entirely viscosity related.



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One point to consider, the viscosity at 100°C may be in the same band for a set of oils of the same high temp classification (0W-30, 5W-30, 10W-30) but the variance in viscosity with temp is different. A 0W-30 will be a bit thicker at the higher temps (100+). I dont worry if my sump temp reaches 120°C but in the bearings the friction will heat the oil to 130-135°C so you need to consider the film thickness at this condition.


The main values I look for in an oil therefore (apart from the latest SAE spec) is the HTHS (high temperature, high shear rate) figure. This is a much more useful indicator of the oils load bearing capability as its measured with the oil at 150°C by a process which shears the oil almost as occurs in the bearings (although at a couple of orders lower rate) I like to see an HTHS figure in the high 3s low 4s. This is met by Mobil1 0W/40, Motul 5W/30 and many others. Casrol 10W/60 and Mobil1 15W/50 are over 5 which is overkill for anything sub R500 loonacy. I tend to see that the higher quality esther based stuff has a better tendancy to hold a high HTHS. Lesser synthetic blends tend to lose viscosity significantly more when tested for HTHS so they dont quote it very often (has anyone got an HTHS figure for Comma Caterham stuff? I bet its of the same order as Mobil1 but it has a base SAE grade of 15W-50 rather than 0W/40 so it will sap energy at lower temps and increase fuel consumption.)


Anyway, I've got to say a big thanks to Oilman because his website lists lots of data which is really useful for my day-to-day analysis and I'll be getting a bulk order from you in the not too distant future.





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Indeed you are right but oil temp is important for selecting the correct sae oil be it 30,40,50 etc.


To illustrate this just look at these figures for a 5w-40 and a 15w-50.


...........................Pro-S 5W/40.............Typ. 15W/50


Visc. at 100C...............14.9.......................18.2


Visc at 110C...............12.06.....................14.53


Visc. at 140C................7.09......................8.25


So, at an ‘oil-cooler’ temp of 110C, the 5W/40 is still thicker than the 15W/50 at 140C.


Once again about optimising the oil and not wasting power through lugging a thicker oil

around than you need to.


HTHS, yes key and in fact RS 10w-60 is on the minumum of 3.7 required to meet ACEA/API.


The thicker the oil, the higher the number should be (higher is always better).


For those that don't understand about HTHS, please read this as it explains what it's all about.




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.




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





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Am I right in suggesting that having high engine temperatures is a vicious circle for oil? My reasoning goes as follows...


Engine temperature is high, therefore high viscosity of oil required. But...

High viscosity of oil creates more friction in the engine thus creating higher temperatures.





1400 Supersport with 6 gears and clamshell wings *smile*

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Simon - You're right on the button apart from one small thing, Castrol RS 0W-40 is HTHS 3.7, 10W-60 is 5.7 if memory serves. They've got a typo on their data sheet for 10W-60, I think they used the same template for both and forgot to change the figure. I've got some Comma data sheets but they dont quote HTHS for the Caterham stuff, I bet its less than 5 though.





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