Film Strength--Base Oils

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Jun 11, 2002
Clarksville, Tennessee
All this talk about Synthetic Vs. Dino, no one comes up with the main difference between the two.

Film Strength=Lubricity.

What would be the film strength of a group I oil?
Group II?

Group III?

Group IV?

Group V?

This might shed some answers why one lubricant is more slippery than the other, but not slippery in the sense as it slides faster, or better, but more on the theory that a higher pressure lubricant will need less additives if it's film strength is higher.

As we know the famous aftermarket additves namely prolong and the like conatin clorinated parrifins, which have a very high film strength, hence the reason they work so well, of coarse all of the harmful side affects are why those lubes don't work.

Lets see if anyone can find the information that I have referred to at the top of this. I truley don't know what the film strenghts of those base stocks are, but I would gander a guess they are not all the same.

[ June 17, 2002, 10:46 AM: Message edited by: msparks ]
good point msparks.

My thoughts are this...

a film of oil is basicly a film of oil that seperates two surfaces with hydrodynamic layer of oil.

film strength would be measured by when the hydrodynamic layer is seperatated and the two surfaces are in direct contact with each other.

So how is the film of any oil sheared or pushed apart to create this..

When the two mating surfaces push against each other, provided that the oil has somewhere else to move to, the oil will move at the same rate as what the viscometric meters had determined. so a 10w30 either synth or mineral showed to flow the same both should basicly flow the same thus the film strength is measured by flow properties. That is why a heavier oil has a slower flow thus a higher film strength unlike a thinner oil.

Given that, film strength is relational to viscosity. The heavier the slower the flow, the higher the film strength.

As for the different groups, groups are VI dependant and is a measure of how well the VI's hold up under certain heated conditions. This relates to the film strength by using the condtions of if the oil flow is affected by heat and the oil thins out at a lower temp, then the viscosity has changed and the flow properties have changed so then the film strength has changed.

This is where over a period of time, the higher vi oils will stay in grade and maintain a consistant film strength and therefore maintain the same flow rate for longer periods of time.

This is the main difference on how a synth over a standard old mineral oil can help with wear. But given new freash oil, both measuring the same cst's, and both measuring the same additive levels, the difference is non existant.
I have checked some of my resources and haven't found any info on that yet. BUT...interesting enough, Terry was telling me about an STLE article on pressure measurements in an engine bearing and was really suprised as to the extremes that they measured. More so than you'd expect.

I can't find my mag so I asked a friend to send me a copy of it. let you know more later.
In the "Motor Oil Bible", the author states that the typical conventional oil (I guess class II) has a shear strength of about 500 psi., whereas a typical sythetic over 5000 psi.. Can this be confirmed???
The one study I read was on pressures verses heat capacity (on both petroleum and synthetics) and they took the oil pressures to about 1 GPa or 145,000 psi.

Unfortunately, they didn't report film strength's, but did report that
esters had the highest heat capacities.

Like Bob, I'm still searching for an article that differentiates dino and synthetic film strengths.
I had heard 750 psi for dinos and 3500 psi for synth's; an approx. 1:5 ratio.

BTW, at extreme high pressures and heats, the oils take on a glass-like phase. I doubt
this happens in commercial engines. (Just free and useless info; didn't cost anything, ha!)
Thread from the dead....
Thanks for the link JAG.

Diesters and polyol esters, because of their polarity, have excellent lubricity and film strength, followed by polyalkylene glycols. PAOs, which are non-polar, have the lowest level of lubricity and film strength of this group. The film strength, coupled with additives, helps minimize wear under boundary lubrication conditions.
That's a great one. Those two sentences say a heck of a lot about how oil works, and it answers some of the questions I've been wondering and asking about. Thanks for posting that JAG and buster. I'll have to read the whole article later when I have time.
I would assume that additives and just adding some amount of ester could significantly alter a base oil's film strength. In other words, standing alone, a POE would have a greater film strength than a PAO but once additives are in the mix and small amount of ester, the film strength would change.
I read somewhere that said polyol esters are up to 90% biodegradable.
Originally Posted By: buster
I read somewhere that said polyol esters are up to 90% biodegradable.
Everything is biodegradeable "eventually"
I am curious though how much additives in a PAO base oil it would take to equal the polarity of a POE base oil? Or does this addition of special additives do nothing for the polarity of the oil and just simply apply to the lubricity of the oil in general? With respect to PAO this adding things to make up for short falls in the base oil seems like the band-aid approach if I am understanding this correctly. Granted it probably makes little to no difference in a street engine. Just interesting.
Until recently I believe it was well understood that PAO oils needed to be mixed with some Grp V to give proper performance, and I suspect that part of the reason for that was the need to have some polar content to provide "stickiness" to the metal. More recently Grp III oils have become available, and we hear a lot about premium synthetics being mixed with Grp III, which most people take to be for cost-reduction but which I imagine has to do with the fact that similar performance can now be achieved with less expensive but still polar base stocks.

I suspect that there is a strong tendency for the polar components in the oil, even if they are fairly small in proportion, to attract themselves to the metal surfaces, and if that is true then the effectiveness of a small polar component would be fairly high and blending in larger amounts of polar base stocks would not give a linear increase in benefit.

I suppose it is a band-aid, but if you want to look at it that way then you have to realize that almost any minority component of any oil is a band-aid, as every component added is added to make up for some weakness in the base stock by itself. No base stock is a good detergent, for example, so detergent additives are added as a band-aid, as are anti-foam, anti-wear, (sometimes) VII, and all other additives.

The point of course is that PAOs have some very desirable qualities and oils blended with them can be made to have all the desirable qualities of a superior engine oil in ample degree that the end result is a high performance product that will last longer in use than lower-group oils.

That still leaves the question of whether a good polar oil is better in some aspects of performance due to its polarity, and whether, if you do not need the particular advantages of a PAO, a conventional (or Grp III) oil might give better actual performance in terms of engine wear. That is a question I've been asking for a while and it seems like at least a reasonable possibility.
The best choice for high performance oils are still your PAO's used in conjunction with certain esters. PAO/Ester blends seem to perform best in most applications.
That being said, what does Amsoil use as far as an ester to supplement the PAO's? I have used POE based oils in a few racing applications with great success. For this reason I use Red Line for my high performance cars and Amsoil for my daily drivers.
Originally Posted By: AzFireGuy79
That being said, what does Amsoil use as far as an ester to supplement the PAO's? I have used POE based oils in a few racing applications with great success. For this reason I use Red Line for my high performance cars and Amsoil for my daily drivers.

I almost had the same question. What oils use PAO/Ester exclusively?
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