Why Oils Lose Viscosity With Use

[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 straucture 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

[Karma Supra]

So in time the VI Improvers break up, which would explain why my (hot) oil presure has steadily decreased the last couple of months, dispite the colder weather and start up presure being very high??

I tried a new type of oil... it was crap :D

[DaveSR]

What about that stuff I keep seeing on QVC ........

Greased Lightning?

Looks good.....any ideas?

[oilman]

What for? the lawnmower!

Cheers

Simon

[DaveSR]

What for? the lawnmower!

Cheers

Simon

←

*cough

A response from anybody not trying to flog the stuff or competitors stuff would be nice!

You calling my yaris a lawnmower hey :P !

[knightrider]

i cant see anything off QVC being any good, 10-40 millers is great makes the car soo much quieter i mean u'd think clarkys was a fresh factory (bar the mods) sweet as a nut at like 80k

[ENVY]

You calling my yaris a lawnmower hey  :P !

:D :P

[Puls]

The Mecca of Motor Oil

[chrisb]

This is useful!

http://www.carbibles.com/engineoil_bible.html

CB

[oilman]

So is this for technical data on hundreds of oils.

http://www.opieoils.co.uk/lubricants.htm

Cheers

[Leeky]

So is this for technical data on hundreds of oils.

http://www.opieoils.co.uk/lubricants.htm

Cheers

←

Im starting to get the impression you are plugging your own site/company just a tad ;)

[oilman]

There is nothing for sale there, it's purely technical data for reference purposes.

I would like to become a trader/sponsor here (as on many sites we post on) and have asked via PM on many occassions but with no response.

I do not expect something for nothing and am happy to support the site as a legitimate trader. Can you help me? PM if you wish.

Cheers

Simon

[nrgizerbunny]

I use Mobil 1 in my lawnmower! my 145cc honda HRB engine purrs like a !Removed! cat.

[Karma Supra]

Not too good on the hills though :(

[jaxx]

There is nothing for sale there, it's purely technical data for reference purposes.

I would like to become a trader/sponsor here (as on many sites we post on) and have asked via PM on many occassions but with no response.

I do not expect something for nothing and am happy to support the site as a legitimate trader. Can you help me? PM if you wish.

Cheers

Simon

←

have ya PMd steve?

[oilman]

I've PM'd 3 different people in the last 9 months and then gave up.

Cheers

Simon