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Why oils lose viscosity with use

 
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oilman
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PostPosted: Tue Oct 18, 2005 1:20 pm    Post subject: Why oils lose viscosity with use Reply with quote Reply with quote including images

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
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Frustrated Pilot




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PostPosted: Tue Oct 18, 2005 1:27 pm    Post subject: Reply with quote Reply with quote including images

=D> fantastic.... again a top quality thread by oil man.

[Embarassed] what should i be using for my tubby ?? [Embarassed]

[Mr. Green]
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Cliff




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PostPosted: Tue Oct 18, 2005 1:36 pm    Post subject: Oil recommendation Reply with quote Reply with quote including images

I use Silkolene Pro S 15W 50 if that helps.
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Frustrated Pilot




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PostPosted: Tue Oct 18, 2005 1:37 pm    Post subject: Reply with quote Reply with quote including images

cheers dude.. whens the next event? pm me
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oilman
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PostPosted: Tue Oct 18, 2005 5:05 pm    Post subject: Reply with quote Reply with quote including images

Silkolene PRO S 10w-50 if it's modded and used on track, if not then PRO S 5w-40.

Cheers
Simon
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Cliff




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PostPosted: Tue Oct 18, 2005 8:23 pm    Post subject: Oil Recommendation Reply with quote Reply with quote including images

oilman wrote:
Silkolene PRO S 10w-50 if it's modded and used on track, if not then PRO S 5w-40.

Cheers
Simon


Oops, just checked and I also use Silkolene Pro S 10W 50.
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Goldy




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PostPosted: Wed Oct 19, 2005 12:05 am    Post subject: Reply with quote Reply with quote including images

is 5w 30 fully synthetic ok to use on an regular n/a engine?
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MR2Mania




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PostPosted: Wed Oct 19, 2005 12:15 am    Post subject: Reply with quote Reply with quote including images

Nice post, oilman.

What do you think of Mobil 1 Motorsport (15w-50)? Also, what do you think of the 76 Racing oils (20w-50)?
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oilman
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PostPosted: Wed Oct 19, 2005 8:35 am    Post subject: Reply with quote Reply with quote including images

Goldy wrote:
is 5w 30 fully synthetic ok to use on an regular n/a engine?


5w-30 or 5w-40 is fine as long as the car is not heavily modded.

Cheers
Simon
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oilman
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PostPosted: Wed Oct 19, 2005 8:36 am    Post subject: Reply with quote Reply with quote including images

MR2Mania wrote:
Nice post, oilman.

What do you think of Mobil 1 Motorsport (15w-50)? Also, what do you think of the 76 Racing oils (20w-50)?


10w-50 or 15w-50 if the car is tracked or heavily modded. 20w-50 is too thick at cold start causing excessive wear.

Cheers
Simon
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DotNetter
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PostPosted: Wed Oct 19, 2005 10:41 am    Post subject: Reply with quote Reply with quote including images

Hi Oilman

Having read this article + the copied article now in the KB, I'm thinking of changing my oil to 0W 40 for the winter months.

This is based on the idea that the lower 0W will provide better protection during cold starts where the engine will take longer to reach temp due to colder weather.

Is this good logic or is the weather of little issue?

I'd also be interested to know how long it takes oil to reach temp rather than the engine coolant which drives the temp gauge (unless it's that temp sensor on the engine bay lid [Think] )

btw, I have a rev1 NA with only 2 mods; a Mongoose exhaust and K&N filter.

Regards


Wayne
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oilman
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PostPosted: Wed Oct 19, 2005 12:52 pm    Post subject: Reply with quote Reply with quote including images

DotNetter wrote:
Hi Oilman

Having read this article + the copied article now in the KB, I'm thinking of changing my oil to 0W 40 for the winter months.

This is based on the idea that the lower 0W will provide better protection during cold starts where the engine will take longer to reach temp due to colder weather.

Is this good logic or is the weather of little issue?

I'd also be interested to know how long it takes oil to reach temp rather than the engine coolant which drives the temp gauge (unless it's that temp sensor on the engine bay lid [Think] )

btw, I have a rev1 NA with only 2 mods; a Mongoose exhaust and K&N filter.

Regards


Wayne


Wayne,

Yes a good 0w-40 or 5w-40 does not matter which is great for good cold start protection and fine for the na, so your thinking along the right track.

It depends on the car, but it only takes about 10 to 15 min to get the oil up to temp.

Cheers.
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