It's a question I get asked all the time so I thought that I'd clear up one of the most common misconceptions about oil viscosity.

"What's the difference between a 10w-40 and a 5w-40?"

I read the following comments all the time!

"You must not use 5w oils"

"5w oils are too thin"

"10w-40 is ok but don't use 5w-40!"

Some facts need to be straightened out here and hopefully I can do this without losing those that are interested.

So what is the difference between a 5w-40 and a 10w-40 @100degC?

NONE!

SAE determines the viscosity (+/-) that the oil needs to be at 100 degC and these need to be met in tests to give the oil it's API rating (xw-xx or xxw-xx).

They are as follows: (within a margin)

SAE 30 = 11.00cst
SAE 40 = 14.00cst
SAE 50 = 18.50cst
SAE 60 = 24.00cst

Compare these SAE 40 oils at 100degC (0w, 5w, 10w, 15w)

Motul 300V 5w-40.................Viscosity = 13.80cst
Motul 300V 10w-40................Viscosity = 14.00cst
Silkolene PRO S 5w-40............Viscosity = 14.89cst
Motul 8100 X-cess 5w-40........Viscosity = 14.00cst
Fuchs Titan Supersyn 5w-40....Viscosity = 13.60cst
Castrol Performance 10w-40.....Viscosity = 14.50cst
Silkolene XTR 10w-40 (semi).....Viscosity = 14.70cst
Mobil 1 0w-40........................Viscosity = 14.30cst
Motul 8100 0w-40...................Viscosity = 13.30cst
Silkolene Turbolene D 15w-40....Viscosity = 14.40cst
Total Quartz 15w-40...............Viscosity = 14.50cst

These figures are not by chance and include synthetics, semi-synthetics and mineral oils they are all in the range to be labelled a Xw or XXw-40 multigrade oil.

So, what's the "W" number all about then?

It stands for "winter" not "weight" as often confused and called on many U.S. articles on oil!

This number is the "cold crank" viscosity and nothing to do with the oil viscosity when the engine is up to temperture. These numbers are related to the oils ability to operate in cold temperatures. 0w oils were originally designed to operate in arctic climates that's why they operate at ferrous monkey endangering temperatures of -35degC and below!

The benefits of the lower viscosity oils (0w and 5w) is that they flow more easily and quickly when cold and therefore protect the engine better on cold start when 80% of the engine wear occurs.

So, yes it's true an sae 40 is an sae 40 when hot whether its a 0w, 5w, 10w, 15w or whatever and that's a fact. The same goes for 20's, 30's, 50's, 60's and so on.

Hope this helps.

Cheers
Simon

 

 

 

That confirms what I always thought

Another top post from the oilmen.

 

there can be only one....

 

wtf

 

No, no, there's two I am sure

Anyway - back on track.

 

Indeed, Simon and (brother) Guy

Cheers
Simon

 

so y does mr ford say 20/50 valvoline the reason they give is it chatters to much on lesser grade oil im thick and like to be enlightend please

jay

 

20w-50 is not going to chatter more than 10w-50 as they're the same when hot.

Ok, 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. You're confused! It's true believe me and here is a table to illustrate this.

SAE 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
Simon

 

Absolutly clear.

Regards.

 

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



cheers oilman thats what i was wondering thanks for your time and knowledge, boy i love this site

jay

 

There is very important factor of oil stability. Not every 40 grade oil is as good at over 100, the same about 50 or 60. Also tehre is oil wear. Some oils after only just couple hundred stage miles can be 40 or 30 out of starting 50. There are some oils that cope better with being diluted by fuel some don't cope with that it all has to be taken into account in high powered turbo-charged motor. Also the thing that oilman said about oil temperatures. It's not only the matter of oil temperature in the sump measured by your temperature sensors but also about local overheats inside the engine. If oil works in exreme condition oil sqirted into pistons will overheat and loose some of it's properties with each combustion cycle and so on. Quite a complex matter, that's why it's worth it to use top oils, they are quite cheap compared to engine internals

 

Azrael,

Well put, could not agree more.

Always go for the best you can afford in the car that you love, with the longer drain intervals of the top quality synthetics, over a period of time they can work out cheaper then their mineral based cousins.

Cheers

 

Yep but I wouldn't change oil less often then 10k kms in a hard used turbo-charged petrol engine. New Scoobies and Evos have longer periods, still if they are hard used the oil which is drained during changed is very very black and dirty. It's worth to change it frequently - e.g. after trackdays.