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Let's clear up a common misconception (oil viscosity)
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oilman
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PostPosted: Wed Jun 01, 2005 12:46 pm    Post subject: Let's clear up a common misconception (oil viscosity) Reply with quote Reply with quote including images

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
Guy
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cantfindausername




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PostPosted: Wed Jun 01, 2005 1:26 pm    Post subject: Reply with quote Reply with quote including images

Useful info for sure. Maybe one for the knowlegde base.

My car is gonna mainly live on the red line as it will be used mainly on the track once its built. So for this what would you recommend after run in?

Thanks
Ant
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oilman
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PostPosted: Wed Jun 01, 2005 4:28 pm    Post subject: Reply with quote Reply with quote including images

If the car is going to spend most of its time on the track I would go for the Silkolene Pro R 15w-50 as the top option.

Tech data here http://www.opieoils.co.uk/lubricants.htm

Cheers

Guy.
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Mark Edwards




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PostPosted: Wed Jun 01, 2005 4:33 pm    Post subject: Reply with quote Reply with quote including images

Thats great and informative info and explains the second number brilliantly, but a bit more about the first number would be interesting too. I get it that for the main use my car gets, that i need to be using a xW 40 oil, but what first number should i use? I usually stick with the stock recommendation of 10w 40 but i don't know why?
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oilman
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PostPosted: Wed Jun 01, 2005 7:53 pm    Post subject: Reply with quote Reply with quote including images

Mark,

The best way to illustrate this is by putting up some more numbers but before I do that, remember that at 0degC, 10degC and 40degC the oil is thicker than at 100degC.

The "w" number actually stands for "winter" and it's the "cold crank viscosity" of the oil.

The thickness of the oil varies dependent on viscosity but, here are some examples.

0w-20..at.......... 0degC = 328......at 10degC = 181......at 100degC = 9
5w-40..at ........................811........................421..........................14
10w-50.at.......................1039........................539.........................18.5
15w-50.at.......................1376........................675.........................18.5
20w-50.at.......................2305.......................1015........................18.5

Obviously the thinner the oil the better the flow at 0degC and 10degC.

If you understand my two posts then you will now be getting the picture of what's best. You need an oil that flows quickly at cold start protecting the engine from wear (5w and 0w do this best) and an oil that will protect your engine when hot depending oil oil temps.

A very good grade for both top and bottom protection is 5w-40 which is very suited to European climates and is more shear stable than 0w-40.

Hope this helps,

Cheers
Simon
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Mark Edwards




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PostPosted: Wed Jun 01, 2005 8:01 pm    Post subject: Reply with quote Reply with quote including images

Yeah, that all makes perfect sense now.

Thanks [Wink]
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raptor95GTS




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PostPosted: Wed Jun 01, 2005 8:35 pm    Post subject: Reply with quote Reply with quote including images

yup, top info Simon =D> . Defo a sticky in my book
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Baker




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PostPosted: Wed Jun 01, 2005 8:48 pm    Post subject: Reply with quote Reply with quote including images

but why does myNA leak when using 0-40, but nothing leaked when i used 15-60? is that not the thickness?
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jonb-




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PostPosted: Wed Jun 01, 2005 8:58 pm    Post subject: Reply with quote Reply with quote including images

Fantastic information... now if you could do what no man has done before and explain the exact differences between torque and bhp you'll get a medal [Wink]
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ENSMR2




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PostPosted: Thu Jun 02, 2005 7:26 am    Post subject: Reply with quote Reply with quote including images

So what does the 5w mean and what does the 40 mean [Confused]
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PostPosted: Thu Jun 02, 2005 8:00 am    Post subject: Reply with quote Reply with quote including images

ENSMR2 wrote:
So what does the 5w mean and what does the 40 mean [Confused]


Thats gotta be a joke [Laughing]

Thanks for all the info. Has definately cleared alot of confussion up.
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ENSMR2




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PostPosted: Thu Jun 02, 2005 8:25 am    Post subject: Reply with quote Reply with quote including images

cantfindausername wrote:
ENSMR2 wrote:
So what does the 5w mean and what does the 40 mean [Confused]


Thats gotta be a joke [Laughing]

Thanks for all the info. Has definately cleared alot of confussion up.


Yes [Laughing] Just thought it funny how the thread starts with

oilman wrote:
I will try to explain it without getting over technical and we'll go from there.


and then started very technical lol.
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BenF
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PostPosted: Thu Jun 02, 2005 10:43 am    Post subject: Reply with quote Reply with quote including images

Great post - now in the Knowledgebase -

http://www.imoc.co.uk/forums/kb.php?mode=article&k=65

(c'mon people, you can submit these articles yourselves you know!) [Wink]
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JohnA
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PostPosted: Thu Jun 02, 2005 2:04 pm    Post subject: Reply with quote Reply with quote including images

After reading some earlier posts I have just bought some Silkolene 10w50 for my rev1 Turbo (fairly standard car). After reading this I think I would be better with a thinner oil....anyone want to buy an unopened 4 litre can of Silkolene PRO S Fully Synthetic 10w50?? 20 including postage.
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PostPosted: Thu Jun 02, 2005 3:51 pm    Post subject: Reply with quote Reply with quote including images

JohnA wrote:
After reading some earlier posts I have just bought some Silkolene 10w50 for my rev1 Turbo (fairly standard car). After reading this I think I would be better with a thinner oil....anyone want to buy an unopened 4 litre can of Silkolene PRO S Fully Synthetic 10w50?? 20 including postage.


No need to change, the 10w-50 is ideal for the turbo model, the na is well suited to the 5w-40.

I would stick with it.

Cheers

Guy.
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PostPosted: Thu Jun 02, 2005 4:01 pm    Post subject: Reply with quote Reply with quote including images

Guy

The car is near enough standard and spends most of it's life doing short journeys, also fuel consumption is horific, from what you were saying a less viscous oil could improve this and also protect better when it's not fully up to temperature.

For the sake of 30 I would prefer to go with the best option, what do you think?

Cheers

John
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PostPosted: Thu Jun 02, 2005 4:14 pm    Post subject: Reply with quote Reply with quote including images

JohnA wrote:
Guy

The car is near enough standard and spends most of it's life doing short journeys, also fuel consumption is horific, from what you were saying a less viscous oil could improve this and also protect better when it's not fully up to temperature.

For the sake of 30 I would prefer to go with the best option, what do you think?

Cheers

John


John,

Its up to you, but I would be happy with the 10w-50.

Cheers

Guy.
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PostPosted: Thu Jun 02, 2005 4:30 pm    Post subject: Reply with quote Reply with quote including images

Baker wrote:
but why does myNA leak when using 0-40, but nothing leaked when i used 15-60? is that not the thickness?


any responce to this?
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oilman
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PostPosted: Thu Jun 02, 2005 4:37 pm    Post subject: Reply with quote Reply with quote including images

Baker wrote:
Baker wrote:
but why does myNA leak when using 0-40, but nothing leaked when i used 15-60? is that not the thickness?


any responce to this?


0w-40 is much thinner accross the whole viscosity range then the 15w-60 so it did not leak because of how thick the 15w-60 is, however that is too thick and you engine will not have much fun pumping it around.

You need to find a happy medium.

A well maintained NA should not leak with a 0w-40.

Cheers

Guy.
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Jonesie
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PostPosted: Fri Jun 03, 2005 5:36 pm    Post subject: Reply with quote Reply with quote including images

ok then,i read most of that,but i'm gona be honest and say i'm just a dumb guy as far as cars go and like to drive em,but dont know too much bout what makes em go.

With this in mind,wots the best oils to put in my rev2 turbo as its due a change and i really wanna make sure its running the best oils i can get.
It only goes on the road and i only push it when im on the motorway and somewhere nice and open,so no need for race spec oils,but something that would b good all round would b very helpfull indeed.
Thanks in advance [Rolling Eyes]
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