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Are 5w and 0w oils too thin?
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Old 14 Jun 2007, 08:34 AM   #1 (permalink)
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Are 5w and 0w oils too thin?

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?


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 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.



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Old 14 Jun 2007, 06:17 PM   #2 (permalink)
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Good stuff to know however still believe misleading on a car specific forum,reading that "some"ppl could interpret the above as the thinner the better when in fact most ppl run 10w50 10w60 or 15w50 reliably, would never recommend anything else in a classic impreza especially, fair enough in the later models maybe its ok,But i`d sooner sacrifice a small amount of bhp for protection anyday......................You know how much these cost to rebuild !!Still believe youre better off with a thicker oil in a scooby though, fair enough at low temp maybe it wont flow aswell but how longs it cold for???mines warm within about 5mins if that, so say your going on a motorway journey where ya gonna sit around 80-90mph-ish for a few hours surely it`d be better to run a thicker oil-ive used silkolene Pro R 15w50 for about the last 25,000mile (6 oil changes) without any issues and will be trying motul 300v next im just curious if theres any noticeable difference not that im unhappy with the pro r.
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Old 15 Jun 2007, 08:32 AM   #3 (permalink)
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I have only ever used Motul and or Silkolene in the Scoobs and or the Evo.

Found both oils to be excellent - although having said that I am about to change the current Motul 300V15w50 out for Silkolene Pros S 5W40 so I am going for a thinner grade of oil
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Old 15 Jun 2007, 11:27 AM   #4 (permalink)
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think mines 10 50, but i'm tempted to lower that due to the oil temps never getting super high, gonna wait till summer and see what happens then.

might i add lower oil temps are due to uprated oil cooler
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Old 16 Jun 2007, 06:54 AM   #5 (permalink)
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be very wary of the 'thinner is better' argument as you will encounter issues particuarly with 'classic' type cars or high h/p impreza's....................

very general info that may be misleading in the wrong context
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Old 17 Jun 2007, 08:33 PM   #6 (permalink)
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Originally Posted by stockcar View Post
be very wary of the 'thinner is better' argument as you will encounter issues particuarly with 'classic' type cars or high h/p impreza's....................

very general info that may be misleading in the wrong context

i need to give you a riung i think mate lol, temps are way down on what they were, i don't see over 95 now, and then sat cruising on the motorway about 80

might be fine as is but could do with finding out tbh
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