Just might be of interest to some of the readers here.
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
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... which is why I'm happier with a car that uses some oil (and therefore gets replenished with at least a little 'fresh' unsheared stuff) than with a car that uses none between services.
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Thanks again Simon for another authoritative contribution.
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Very informative!
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.
I suppose this is the main reason why extended service intervals are possible?
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Mike Farrow
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That is correct Mike and the reason why most Longlife oils (BMW, VAG etc) are narrow viscosity synthetics like 0w-30.
Cheers
Simon
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Another reason as to why I continue to use the recommended oil in my Alfa (Selenia 20K 10w-40) that is synthetic based and so presumably has some mineral oil in it.
I cannot justify using a cheap alternative espcially as nothing has gone wrong with the engine, ever, in over 120k miles.
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As a side issue, what is so special about the 506 spec oils for the VaG PD diesel engines. This seems to be the highest spec oil known for any manufacturer, and I was wondering if anyone knew why they need such a bonkers spec of oil for these engines ?
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The VW pd engines are very severe on the oil.
The mechanical fuel pump delivers fuel at approx. 25,000 psi !!!!! to enable a very precise injector spray pattern which, in turn, gives excellent driving characteristics.
The very high loads on the fuel pump will lead to VI improver shear down and oil viscosity loss if the engine is operated on lesser oils.
Cheers
Simon
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Thanks for the reply, I guessed it was something to do with the cam-driven unit injectors on these engines.
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