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

Simon

Thank you.

OIL bear that in mind........

(crap joke alert)

Frying fish and chips makes oil lose it's viscosity too

can I have that in english please.

E.G

When it is warm it is runny.

When it to too warm it is to runny.

when it is cold it is gloopy

When it is too cold it is like treacle.

Am I close yet?

Cheers

Lee

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' date=' 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

Simon[/quote']

Thanks but my brain hurts now! What, Simon, in your opinion is the best oil to use for a Fabia VRS? I know it's a question thatt's asked loads but you seem to know what your taking about!

Thanks but my brain hurts now! What, Simon, in your opinion is the best oil to use for a Fabia VRS? I know it's a question thatt's asked loads but you seem to know what your taking about!

Well with the VRS your kind of limited.

Because of the unit injecter (pd) you need an oil thats meets the VW505.01 spec, this counts out a lot or "performance" oils.

All VW505.01 spec oils are going to be similar in quality, due to the stringent spec called for by the VAG group.

So, IMO its case of finding an oil that meets this spec for a good price.

Cheers

Guy.

can I have that in english please.

E.G

When it is warm it is runny.

When it to too warm it is to runny.

when it is cold it is gloopy

When it is too cold it is like treacle.

Am I close yet?

Cheers

Lee

Nope

When it's cold it would be thick but by clever wizardry modern oils are still thin

When it's hot it's runny, but not as runny as it would be without the clever mumbo-jumbo

When it's old and been chopped up by an engine for a few thousand miles it gets runnier than it used to when it was new

The wider the difference between the two numbers on the can, the quicker it will go runny with use.

That help?

Nice one

Cheers

Simon

Well with the VRS your kind of limited.

Because of the unit injecter (pd) you need an oil thats meets the VW505.01 spec' date=' this counts out a lot or "performance" oils.

All VW505.01 spec oils are going to be similar in quality, due to the stringent spec called for by the VAG group.

So, IMO its case of finding an oil that meets this spec for a good price.

Cheers

Guy.[/quote']

Thanks for the advice!

...heres some mad information...

the ohc Ford Pinto engine that was fitted to the 2l cortinas, RS2000 escorts etc always had a reputation for destroying camshafts.

In fact the cam and finger rocker follower arrangement was so agressive it litterally tore oil molecules appart and at one time it was the industry std test rig for seeing how long an oil could survive before it did not lubricate proprly anymore!

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' date=' 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

Simon[/quote']

Yeah I know

Actually Nick it does.

especially the bit about the 2 numbers. Always wondered what they were for.

cheers

Lee

Nope

When it's cold it would be thick but by clever wizardry modern oils are still thin

When it's hot it's runny' date=' but not as runny as it would be without the clever mumbo-jumbo

When it's old and been chopped up by an engine for a few thousand miles it gets runnier than it used to when it was new

The wider the difference between the two numbers on the can, the quicker it will go runny with use.

That help? [/quote']

Actually Nick it does.

especially the bit about the 2 numbers. Always wondered what they were for.

cheers

Lee

Er - well actually the bit about the 2 numbers is a side-effect of the mumbo-jumbo. Now we need to get a teeny bit deeper:

Oil is thick when cold and runny when hot

For this reason people used to use thin oil in the winter and thicker oil in the summer

Then multigrade oils were invented with 2 numbers on the can - these use the clever mumbo-jumbo referred to above to behave like BOTH a thin and a thick oil - i.e. they're runnier when cold, and not as runny when hot.

However the clever chemicals that make them do this wear out - hence oilman's original post.

The harder the oil is trying to pretend to be thick AND thin, the quicker it wears out.

Here endeth lesson 2

Nice explanation Nick but perhaps a few numbers will demostrate this.

Here are the figures for viscosity in cst (centistokes) and temperature for different oils.

The first numbers are sae numbers for straight 30,40,50 and 60. The second set of numbers is for various multigrades as they are obviously thinner when cold than straight oils due to lighter viscosity basestocks being used.

Monogrades

DegC...........0.....20......40......60......100......120

Sae 30....1600....315.....95.......39......11........7...

Sae 40....2579....473....135.......52......14.......9....

Sae 50....4592....771....205.......75......18.......11..

Sae 60....7865...1218...303.......105.....24.......14..

Multigrades

DegC...............0...............10............40..............100

0W/20............329............181...........46...............9..

5W/40............811............421...........92..............14..

10W/50..........1039...........539..........117.............18..

15W/50..........1376...........675..........130.............18..

20W/50..........2305..........1015..........148.............18..

These two graphs demonstrate the fact that a monograde is the same thickness at 100degC as a multigrade of the same sae number but the distinct benefits that a multigrade brings at lower temperatures. This is obviously benefits cold crank wear as the rate of flow of multigrades is much better at lower temps.

Cheers

Simon

I think we both need to keep posting Simon. Not everybody wants numbers Sorry to have semi-hijacked your thread though

I agree but the numbers illustrate how multigrades work at lower temps against their monograde grandfathers!

Cheers

Simon