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 Posted: Fri Sep 13, 2013 10:34 am |
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Joined: Feb 17, 2010
Posts: 389
Trade Rating: 0
Location: Sunny Cornwall
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Engine Oil Viscosity
Viscosity is the most misunderstood aspect of oil and yet it is the most important.
Viscosity is the force required to shear (break) the oil at a certain speed and temperature. Oils work because they have viscosity; the drag of a rotating part pulls oil from a low-pressure area into a high pressure area and “floats” the surfaces apart. This is called “hydrodynamic lubrication” and crankbearings depend on it.
Oil must be capable of flowing at low temperatures, so that it gets around the engine in a fraction of a second at start-up and must protect engine components at high temperatures without evaporating or carbonising and maintain adequate (not excessive) oil pressure. Many people think that the thicker the oil, the better the protection, but if the oil is too thick, it will not flow properly, leading to reduced protection.
The numbers on every can of oil indicate its performance characteristics when new but there are many misconceptions on what these numbers actually mean.
For multigrade oils you will see two numbers (for monograde oils only one). The first is followed by a “w” and is commonly 0, 5, 10, 15 or 20. The second number is always higher than the first and is commonly 20, 30, 40, 50 or 60. The first and second numbers ARE NOT related.
The “w” number (0, 5, 10, 15 or 20)
When multigrade oils first appeared, a low temperature test called “w” (meaning “winter” not weight) was introduced. Using a “Cold Crank Simulator", the test measures the oils ability to flow at low temperatures. ALL oils are THICKER at low temperatures than at high temperatures but the lower the “w” number, the quicker the oil will flow at low temperatures.
The second number (20, 30, 40, 50 or 60)
This number is known as the SAE (Society of Automotive Engineers) number and is measured in “Centistokes” (cst) at 100C. 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 thicker the oil.
An oils cst at 100C determines its SAE rating within the following parameters.
SAE 20 = 5.6 to less than 9.3cst
SAE 30 = 9.3 to less than 12.5cst
SAE 40 = 12.5 to less than 16.3cst
SAE 50 = 16.3 to less than 21.9cst
SAE 60 = 21.9 to less than 26.0cst
ALL oils labelled 40 must fall within the SAE parameters at 100C so everything from a monograde 40 to multigrade 0w-40, 5w-40, 10w-40, 15w-40 or 20w-40 are approximately the same thickness at 100C.
Some oil companies label oils as SAE 35, 45 or 55, but as you can see from the above figures, there isn't a SAE 35, 45 or 55. This "could" be because they are approximately on the boundary of the two grades, but as we don't deal with any of those I can't really comment further.
Summary
Cold start.
A 5w-40 will flow better than a 10w-40.
A 10w-50 will flow better than a 15w-50
A 5w-40 is the same as a 5w-30
At operating temperatures.
A 10w-50 is thicker than a 10w-40.
A 15w-50 is thicker than a 5w-40
A 0w-40 is the same as a 10w-40
If you look above, you will see that the figures quoted do not indicate at all as to whether the oil is synthetic or mineral based... Well except for 0w oils as synthetic PAO basestock is required to acheive this viscosity.
Generally the oil you use should be based on the manufacturers recommendation found in the owners manual, but then modifications, climate and the type of use can affect that recommendation. If you are unsure of what is the correct recommendation for your car and would like to know more please contact us here oilman @ opieoils.co.uk
With thanks to John Rowland of Fuchs/Silkolene
Cheers
Tim and the Opie Oils team
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Posted: Fri Sep 13, 2013 1:25 pm |
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Joined: Jun 28, 2012
Posts: 540
Trade Rating: +2
Location: Torbay, England
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This should be a sticky i reckon!
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Posted: Sat Sep 14, 2013 5:24 pm |
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Joined: Feb 10, 2010
Posts: 4266
Trade Rating: +4
Location: Palestine
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Very helpful thread 
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Posted: Sun Sep 15, 2013 9:37 am |
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Joined: Feb 17, 2010
Posts: 389
Trade Rating: 0
Location: Sunny Cornwall
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Glad you like it
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Posted: Sun Sep 15, 2013 9:51 am |
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Joined: Nov 27, 2010
Posts: 11519
Trade Rating: +10
Location: What's it to you? ? ?
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| DanT wrote: |
| This should be a sticky i reckon! |
Totaly pointless as 99% of the forum users already ignore all the other sticky threads people spent time writing 
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| Toyota C-HR GR Sport 2.0 Hybrid with JBL & Alcantara packs. | |
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Posted: Sun Sep 15, 2013 5:41 pm |
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Joined: Jun 28, 2012
Posts: 540
Trade Rating: +2
Location: Torbay, England
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| MrBSI wrote: |
| DanT wrote: |
| This should be a sticky i reckon! |
Totaly pointless as 99% of the forum users already ignore all the other sticky threads people spent time writing |
Well thats because 99% of people don't understand how a forum works
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Posted: Mon Sep 16, 2013 7:26 am |
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Joined: Sep 16, 2011
Posts: 32
Trade Rating: 0
Location: Devon UK
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Thanks great post. I'm a mechanical engineer and kinda knew a bit what the numbers meant but now I know exactly.
Second thing I have learnt is what a sticky is! I was in that 99% but a quick look in the forum FAQ's and soon found what it is, and yes I agree it should be.
Well done again.
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Posted: Thu Sep 19, 2013 12:18 pm |
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Joined: Feb 07, 2010
Posts: 2949
Trade Rating: +6
Location: Athens, Greece
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Absolutely great info!
Best post yet from Opie Oils, thanks very much Tim.
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| 1.4i, 2001, 3-door, China Blue
Repair safely - Drive safely | |
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