10w-40, What does it mean??

[oilman]

We have answered thousands of oil questions in the last 4 years but the most frequently asked one is “What do the numbers mean?”

In short, if you see an expression such as 10W-40, the oil is a multigrade which simply means that the oil falls into 2 viscosity grades, in this case 10W and 40.

This is made possible by the inclusion of a polymer, a component which slows down the rate of thinning as the oil warms up and slows down the rate of thickening as the oil cools down.

Multigrades were first developed some 50 years ago to avoid the routine of using a thinner oil in winter and a thicker oil in summer.

For a 10w-40 to attain the specification target a 10W (W = Winter please note!) the oil must have a certain maximum viscosity at low temperature. The actual viscosity and the temperature vary with the viscosity grade but in all cases the lower the number, the thinner the oil. For example a 5W oil is thinner than a 10W oil at temperatures encountered in UK winter conditions.

This is important because a thinner oil will circulate faster on cold start, affording better engine protection and therefore lower long term wear!

For a 10w-40 to attain the other specification target a “40” oil must fall within certain limits at 100 degC. In this case the temperature target does not vary with the viscosity grade, if there is no "W" the measuring temperature is always 100degC.
Again the lower the number the thinner the oil, a “30” oil is thinner than a “40” oil at 100 degC, which is typical of maximum bulk oil temperatures in an operating engine.

Engine makers are, of course, very well aware of this and specify oils according to engine design features, oil pump capacities, manufacturing tolerances, ambient temperature conditions etc. It is important to follow these guidelines, they are important and are stipulated for good reasons.

Finally, if the engine has been modified or is used in stressed conditions, the operating conditions may well be outside the original design envelope. The stress on the oil caused by increased maximum revs, power output and temperature may require that an oil of a different type and viscosity grade would be required.

These examples show viscosities at different temperatures:

Grade................0degC............10degC...... .........40degC...............100degC

0w-40...............665cst.............354cst........ ..........82cst................14cst
5w-40...............842cst............ 430cst..................91cst................14cst
10w-40.............874cst.............440cst .................91cst................14cst
15w-40...........1260cst.............595cst........... ......107cst................14cst

In a nutshell, that’s what a multigrade is all about!

Guy & the Opieoils.co.uk Team

[s2_bo]

Thanks for that info!

[Error404]

Guy, how is it that I notice a significant difference in engine noise between 5w-40 and 10w-40 when warming up in the temperatures we have had recently?
(Very tappety and generally sounds like a bag of spanners on 5w-40)
From your example figures above there is very little difference between the two

[oilman]

There os a difference at cold only as the 5w is thinner and flows faster

5w is better than 10w for cold start wear and 90% of wear occurs at cold

cheers

[Keith]

Hello,
One important thing is to know which base oil is used.
Mineral : Mineral oils (natural hydrocarbons)

Semi-synthetic : Mixture of Mineral oil ant synthetic oil (usually both hydrocarbon)

100% synthetic : usually a mixture of PAO (Polyalphaolefin oil or SHC as for Synthetic hydrocarbons oil) and esters
We should use these oils for better lifetime and stability.

The Mineral or PAO (SHC) oils are both Hydrocarbon base oils. This means they are carbon based oils.
When submitted to high temperatures (above 80°C) the molecular structure cracks letting carbon residues into the engine.

The ester oils can hold high temperatures for long time with low evaporation and extremely low residues (non carbon based oil).

So the better the oil is the Highest Viscosity index you get.

The viscosity index (VI) is the viscosity behavior in temperature.
All liquids have their viscosity falling with rising temperature.

There is many ways to qualify viscosity :
Multi grade : ex. 15W40
Mono grade : cSt (centistokes) or mm²/s
and others...

so one of the most important thing to choose is the highest possible VI, witch will bring the best stability for temperature changes.

If you have any questions concerning lubricants, I can help. I work for a High-Tech lubricant company

[oilman]

Interesting post, who do you work for if you dont mind me asking?

Cheers

Guy.

[Keith]

German company : Klüber Lubrication
We offer our services to industry professionals.

http://www.klueber.com/

see you.

[CliveH]

Finally, if the engine has been modified or is used in stressed conditions, the operating conditions may well be outside the original design envelope. The stress on the oil caused by increased maximum revs, power output and temperature may require that an oil of a different type and viscosity grade would be required.

Does this mean that a "thicker" or "thinner" oil should be used for such modified engines?

[Keith]

Does this mean that a "thicker" or "thinner" oil should be used for such modified engines?

well yes in theory !

the highest rpm you have the lower viscosity you should use !
the lubricating film will be regenerated faster with low viscosity than with high ones.
The faster your go the faster you need to regenerate the lubricating film.

[CliveH]

well yes in theory !

so which - "thinner" or "thicker"?

[Keith]

so which - "thinner" or "thicker"?

whet do you mean by "thinner" or "thicker" ????

the higher the viscosity is, the more "pasty" a liquid seems.
the lower the viscosity is, the more "liquid" or "fluid" a liquid seems.

does that answer ?

[oilman]

Oil temps will play a part here as well.

Do not use a thick oil such as a 10w-60 if your oil temps never exceed 100degC as this oil is around 70% thicker than the recommended viscosity for your car.

Look to use a low viscosity oil unless totally unavaoidable. Ensure that you use a quality one with a good film strength, preferably a proper synthetic pao/ester and not a mineral based one.

Read on.............................next post.

Cheers

[oilman]

Why do oils lose viscosity with use?

Viscosity Index Improvers.

An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.

VI Improvers are heat sensitive long chain, high molecular weight polymers that minimise the viscosity loss of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.

"Shearing"

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. As with many items the more you pay, the better the finished article and more expensive, usually synthetic oils are likely to incorporate better VI improvers. All other things being equal the less VI improver an oil contains, the better it will stay in grade by resisting viscosity loss.

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 span" 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 span" 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 have a higher inherent VI to begin with, this is due to differences in the molecular structure of the synthetic base oils compared to mineral oils.

It is a fact that many synthetics require significantly less VI Improver to work as a multigrade and are therefore less prone to viscosty loss by shearing.


Cheers

[Jamo]

What about 5w50 Mobil in Performance S2 application?

Typical Properties

Mobil 1 5W-50
Value
Viscosity, ASTM D 445
cSt @ 40º C 104.6
cSt @ 100º C 17.4
Sulfated Ash, wt%, ASTM D 874 1.3
Phosphorous 0.1
Flash Point, ºC, ASTM D 92 228
Density @15º C kg/l, ASTM D 4052 0.86
Total Base Number (TBN) 11.8
MRV at -35ºC 27653
Viscosity Index 184
HTHS Viscosity, mPa•s @ 150ºC, ASTM D 4683 4.3

[Keith]

Why do oils lose viscosity with use?

Viscosity Index Improvers.

An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.

VI Improvers are heat sensitive long chain, high molecular weight polymers that minimise the viscosity loss of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.

"Shearing"

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. As with many items the more you pay, the better the finished article and more expensive, usually synthetic oils are likely to incorporate better VI improvers. All other things being equal the less VI improver an oil contains, the better it will stay in grade by resisting viscosity loss.

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 span" 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 span" 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 have a higher inherent VI to begin with, this is due to differences in the molecular structure of the synthetic base oils compared to mineral oils.

It is a fact that many synthetics require significantly less VI Improver to work as a multigrade and are therefore less prone to viscosty loss by shearing.


Cheers

I'm OK with all that

We should never forget the fact that VI improvers have "lifetime", I mean they crack with time, that's why when an application needs wide range temperature we prefer use ester/pao or even Pag or pfpe (not used in engines) because of their high native VI that will never change in time.

The problem is that the car oil is never at the same temperature, so we should use two viscosities every year (one for winter and one for summer)....the problem is that I only change oil every 12 or 18 month.
So we have to find a middle point able to lubricate as well for highers summer temperatures and starting cold winter temperatures.