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#3393212 - 06/09/14 05:03 PM Synthetic Claims
DWC28 Online   content


Registered: 07/31/03
Posts: 115
Loc: Houston, TX
Getting bored and looking for some lively discussion.
I know that some synthetic oil producers have stated that synthetics are more “slippery” or have lower coefficient of friction than petroleum oils. However I would like to disagree with that statement. Based on many fuel economy tests, I know that it is easier to pass the fuel economy testing with low viscosity petroleum oil than a similar low viscosity synthetic. The problem is you cannot pass volatility requirements with those petroleum oils, thus you cannot use to make motor oils.
Think about testing the viscosity of an 8 cSt PAO and an 8 cSt mineral oil. They both flow through the test tube at the same rate, and thus are the same slipperiness. (This only applies to hydrodynamic regime, but the additives are more important than the base oil in other regimes). If you heat both oils to 150 C you will find the petroleum flows much faster, i.e. it is slipperier. At cold temperatures, the PAO is better. So if you want to know which is more slippery, you need to know the viscosity at the operating conditions. It may be either. By the way there are about 2 million different molecules in 8 cSt PAO. And they all look like barbed wire.

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#3393244 - 06/09/14 05:52 PM Re: Synthetic Claims [Re: DWC28]
Garak Offline


Registered: 12/05/09
Posts: 17994
Loc: Regina, Saskatchewan, Canada
Originally Posted By: DWC28
Think about testing the viscosity of an 8 cSt PAO and an 8 cSt mineral oil. They both flow through the test tube at the same rate, and thus are the same slipperiness.

Viscosity is a scalar. The coefficient of friction is a dimensionless number. I don't think we can find a mathematical relationship between the two, let alone equate the two principles.
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#3393249 - 06/09/14 05:56 PM Re: Synthetic Claims [Re: DWC28]
Triton_330 Offline


Registered: 12/24/13
Posts: 515
Loc: Illinois, USA
Well, I am no chemist, but, let's try to use logic to determine plausible answers here.

Oils in general, not only motor oils, are obviously naturally "slippery" - or rather "oily." With motor oils, conventional oil is refined from true crude oil, and thus, redundancy intended, it is naturally slippery. Synthetic oils (true synthetics, that is) are not made from crude oil, but rather have different base stocks. Safely assuming that chemists [obviously] know what they're doing, the base stocks for synthetic oils are going to last longer strength wise (ergo, won't break down as quickly) than conventional oil's base stock. However, conventional oil has come along way (cliche as that line is, it's true). As far as comparing "slipperiness," a better term would be lubricity.

As you said, synthetics are better lubricants when cold. But, your overarching question is really about whether or not conventional oils are better lubricants when hot (as far as lubricity goes, and excluding strength). I would say that, if that is true, it would make sense... because to me, [true] synthetic [motor] oil isn't really oil at all, seeing as it's not actually made from crude oil. Even if conventional oil base stock doesn't have as much strength as synthetic base stock, it would seem to make sense that a true oil (ergo, conventional oil) would be a better lubricant when hot. Oil is naturally slick, slippery, whatever term have you... so that's my logic.

BUT you are mistaken on one thing for sure.
Originally Posted By: DWC28
So if you want to know which is more slippery, you need to know the viscosity at the operating conditions.

That ^ is wrong. Film thickness is more relevant to lubricity than is viscosity. But really, lubricity is just a whole 'nother property on its own, even though it may be directly and/or indirectly affected by other properties. You can't say "any (or all) 5w-20 oils are more lubricious than any (or all) 5w-30 oils" etc., that doesn't even make sense.

Why not? Because additives play a larger role than you might realize. Many different elements and chemicals are used as oil additives. Just look at PQIA for some VOA's, and you'll see many additives. They serve many purposes, some help with cleaning, others with wear, others with viscosity, friction modifiers, so on and so forth...

So, push come to shove, what is it really even worth discussing when, adding everything together, the logic proves that some conventional oils may be better lubricants than some synthetics, and vice versa.
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#3393265 - 06/09/14 06:12 PM Re: Synthetic Claims [Re: DWC28]
DWC28 Online   content


Registered: 07/31/03
Posts: 115
Loc: Houston, TX
Look up the Stribeck curve which charts coefficient of friction versus viscosity, load and speed. If you hold speed and load constant the coefficient varies direstly with the viscosity.
Sir Isaac a long time ago related the force needed to move an oil film. He found the force (constant area) was related to the shear (speed/film thickness) by a constant that he called viscosity. That is the lower the viscosity the faster he could move the object with the same force.

I prefer to talk about lubricity as a term for use under boundary conditions. There the additives lay down a film much like a shag carpet to seperate the surfaces. Under full film thickness, only the viscosity makes a difference.

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#3393270 - 06/09/14 06:22 PM Re: Synthetic Claims [Re: DWC28]
Kuato Offline


Registered: 10/23/05
Posts: 4844
Loc: In transit...
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#3393277 - 06/09/14 06:25 PM Re: Synthetic Claims [Re: DWC28]
MolaKule Offline


Registered: 06/05/02
Posts: 16954
Loc: Iowegia - USA
Originally Posted By: DWC28
... By the way there are about 2 million different molecules in 8 cSt PAO. And they all look like barbed wire...


They look more like chicken wire or Cyclone fences.

What are you referring to when you say 2 million "different" molecules?

http://utsrus.com/documents/seminary_doklady/exxon_mobil_pao.pdf


Edited by MolaKule (06/09/14 06:26 PM)
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#3393296 - 06/09/14 06:41 PM Re: Synthetic Claims [Re: DWC28]
Garak Offline


Registered: 12/05/09
Posts: 17994
Loc: Regina, Saskatchewan, Canada
Originally Posted By: DWC28
Look up the Stribeck curve which charts coefficient of friction versus viscosity, load and speed. If you hold speed and load constant the coefficient varies direstly with the viscosity.

My wording wasn't very elegant. Nonetheless, take a look at a curve again. The relationship isn't direct at all. My whole point was that there are different relationships depending upon the regime. And, considering that engines experience them all at one point or another, you're not going to get a nice tidy relationship.
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2008 Infiniti G37 coupe - Mobil Delvac 1 ESP 5w-40, Wix 51358
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#3393338 - 06/09/14 07:06 PM Re: Synthetic Claims [Re: DWC28]
MolaKule Offline


Registered: 06/05/02
Posts: 16954
Loc: Iowegia - USA
Originally Posted By: DWC28
Look up the Stribeck curve which charts coefficient of friction versus viscosity, load and speed. If you hold speed and load constant the coefficient varies direstly with the viscosity.
Sir Isaac a long time ago related the force needed to move an oil film. He found the force (constant area) was related to the shear (speed/film thickness) by a constant that he called viscosity. That is the lower the viscosity the faster he could move the object with the same force.



I prefer to talk about lubricity as a term for use under boundary conditions. There the additives lay down a film much like a shag carpet to seperate the surfaces. Under full film thickness, only the viscosity makes a difference.


Recommend you stick to one topic at a time.

What is your main hypothesis?
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#3393379 - 06/09/14 07:54 PM Re: Synthetic Claims [Re: MolaKule]
DWC28 Online   content


Registered: 07/31/03
Posts: 115
Loc: Houston, TX
Regarding
What is your main hypothesis?
1. Synthetics are not uniform molecules like golf balls. There are millions of different ones in a PAO 8 cSt. Granted most have carbon numbers of about 80 but there are isomers galore in different shapes.
2. I am only talking about PAOs as there are many other synthetics.
3. The fuel efficiency tests for motor oils are nothing more than viscosimeters. All 0W-20s will be more fuel efficient than 10W-30s.
4. I like to read the wisdom on this board, and perhaps change some opinions.
5. I roam around as I am ADD.

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#3393527 - 06/09/14 10:02 PM Re: Synthetic Claims [Re: DWC28]
4wheeldog Offline


Registered: 04/23/12
Posts: 1390
Loc: East Mountains, NM
I think that most of the efficiency improvement with the use of synthetic oil has more to do with better ring sealing than it does less friction. Just sayin'......But it does not fit with the OP's hypothesis.

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#3393561 - 06/09/14 11:13 PM Re: Synthetic Claims [Re: DWC28]
HTSS_TR Online   content


Registered: 04/17/06
Posts: 17885
Loc: Lake Forest, CA
One of main advantages of synthetic is longer OCI. Mercedes learned that lesson 13-14 years ago, it costs them about $100 mils to settle class action lawsuit for sludge engines with conventional in their engine, when owners followed the FFS (MB oil life monitor). They should use synthetic, but OM didn't clearly state that.
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