Why do oils lose viscosity with use?
![](./images/smilies/spacer.gif)
Viscosity Index Improvers.
![](./images/smilies/spacer.gif)
An oils viscosity will decrease as the engine temperature rises.
![](./images/smilies/spacer.gif)
Viscosity Index Improvers are added to reduce this thinning.
![](./images/smilies/spacer.gif)
They are a key addative in the production of multigrade oils.
![](./images/smilies/spacer.gif)
VI Improvers are heat sensitive long chain, high molecular weight polymers that minimise the viscosity loss of the oil at high temperatures.
![](./images/smilies/spacer.gif)
They work like springs, coiled at low temperatures and uncoiling at high temperatures.
![](./images/smilies/spacer.gif)
This makes the molecules larger
![](./images/smilies/spacer.gif)
(at high temps) which increases internal resistance within the thinning oil.
![](./images/smilies/spacer.gif)
They in effect
![](./images/smilies/spacer.gif)
"fight back" against the viscosity loss in the oil.
![](./images/smilies/spacer.gif)
"Shearing"
![](./images/smilies/spacer.gif)
The long chain molecules in VI Improvers are prone to
![](./images/smilies/spacer.gif)
"shearing" with use which reduces their ability to prevent the oil from losing viscosity.
![](./images/smilies/spacer.gif)
This
![](./images/smilies/spacer.gif)
"shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules.
![](./images/smilies/spacer.gif)
The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.
![](./images/smilies/spacer.gif)
This shearing not only reduces the viscosity of the oil but can cause piston ring sticking
![](./images/smilies/spacer.gif)
(due to deposits), increased oil consumption and increased engine wear.
![](./images/smilies/spacer.gif)
Like basestock quality, VI Improvers also vary in quality.
![](./images/smilies/spacer.gif)
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.
![](./images/smilies/spacer.gif)
All other things being equal the less VI improver an oil contains, the better it will stay in grade by resisting viscosity loss.
![](./images/smilies/spacer.gif)
Which oils require more VI Improvers?
![](./images/smilies/spacer.gif)
There are two scenarios where large amounts of these polymers are required as a rule.
![](./images/smilies/spacer.gif)
Firstly in
![](./images/smilies/spacer.gif)
"wide viscosity span" multigrades.
![](./images/smilies/spacer.gif)
By this I mean that the difference between the lower
![](./images/smilies/spacer.gif)
"W" number and the higher number is large for example 5w-50
![](./images/smilies/spacer.gif)
(diff 45) and 10w-60
![](./images/smilies/spacer.gif)
(diff 50) are what is termed as
![](./images/smilies/spacer.gif)
"wide viscosity span" oils.
![](./images/smilies/spacer.gif)
Narrow viscosity oils like 0w-30
![](./images/smilies/spacer.gif)
(diff 30) or 5w-40
![](./images/smilies/spacer.gif)
(diff 35) require far less VI Improvers and therefore are less prone to
![](./images/smilies/spacer.gif)
"shearing".
![](./images/smilies/spacer.gif)
Secondly, mineral and hydrocracked
![](./images/smilies/spacer.gif)
(petroleum synthetic oils) require
![](./images/smilies/spacer.gif)
more VI Improvers than proper PAO/Ester
![](./images/smilies/spacer.gif)
(Group IV or V) synthetic oils
![](./images/smilies/spacer.gif)
as they have a higher inherent VI to begin with, this is due to
![](./images/smilies/spacer.gif)
differences in the molecular straucture of the synthetic base oils
![](./images/smilies/spacer.gif)
compared to mineral oils
![](./images/smilies/spacer.gif)
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.
![](./images/smilies/spacer.gif)
Cheers
Simon