Is viscosity a linear relationship?

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Oct 22, 2003
By Detroit
Is is safe to assume that viscosity is a linear relationship, that is, is a 16 cSt twice as thick as an 8 cSt? Or is is just that the 16 cSt is twice as slow through the viscometer than the 8 cSt and maybe only 50% thicker. Trying to understand what it really means to go from, say a 10.5 cSt 30 weight to a 14 cSt 40 weight.
I don't really know the answer, but the cS reading is obtained on a dynamic viscometer, i.e., there is a disk placed in the oil and the resistance to turning is read out on the instrument. It is almost like a torque reading in a way. (?) SSU is a measurement of oil flowing through an oriface at a specific temperature and it is measured in seconds. It has been a while since I have dealt with these, but that is what I remember from what we had in a power plant laboratory.
In a previous post I typed cS, should have been cSt. If you do a search on kinematic viscosity you get several good hits and calculators. I get concerned when people interchange the word thickness and viscosity, as many on this site do.... I am not sure how they relate, I know that a high viscosity oil flows thicker, but I am not sure that in a hot engine it results in a thicker film since there are other constraints, i.e., clearances and such... I have enough knowledge to get in trouble here.... not enough to answer absolutely..........hope I'm not adding to confusion.
I am one who has tried different oils in my cars. I have gone as little as 100-200 miles then switched to another. Oil is relatively cheep to experiment with. I have gone as little as 50 miles on a gear oil.

For example, in my Maranello I went from a 40 to a 20 wt. synthetic. The oil pressure dropped only 10 PSI or less at 2,000 RPM - from around 80 to around 70-75. The temperature dropped 25 degrees however.

If the oil was half as thick one would think the pressure should drop to 40 PSI. After all who cares what the lab value is, engine conditions are what count.

There are different views for sure but the litmus test is in your engine and with as many ways to test the oil as possible including but not limited to seat of the pants, MPG, consumption, BHP, and wear analysis.


Let’s looks at some numbers. The following are viscosities at 40, 100, 150 C.
Compare Red Line 5-30 to other 5W-30 oils:


..58..(thinner)..10.7..(thicker)..3.07.(thinner)..Penn. Synth.
..66..(thicker)..10.9..(thicker)..3.00.(thinner)..Valvoline Syn. Pwer.
..63..(thinner)..11.7..(thicker)..3.50.(thinner)..Amsoil ASL

If one of the above is supposed to be linear then the other three cannot be linear.

Somewhere ASTM has the specs for Gardner-Holt viscosity tubes. They are maybe 3/8'' ID and about 6'' long. They have lines etched on them top and bottom. Fill one with a liquid leaving about a 1/2'' bubble. Time it from one mark to the next. It has been 30 years since I have. I forget whether the time in seconds was stokes or centistokes. Poise is the product of stokes times the specific gravity. A 16 cst oil would take twice as long to rise to the top as an 8 cst one.

I think viscosity cups give about the same readings for liquids that have the same viscosity over a range of shear. For many liquids, the viscosity goes down as shear goes up, some more so than others. Great in paint, but not a lube.
Some oils of the same viscosity are vastly different in high shear stress conditions. One can compare a mineral vs a synthetic of the same viscosity.

Also, a synthetic may have the same viscosity as a mineral oil but has a higher film strength. This may again act as a thicker oil even though the "test" gives the same value.

This certainly contributes to the statements made by many people that two different oils, even with the same exact viscosities, act very different in a single engine. Obviously there are the effects of the additive package but all things must contribute and going solely by the numbers does not tell the whole story.

How about fluidity. Synthetics have more fluidity than mineral oils of the same viscosity. This is a diminished internal resistance. One could argue that this is one area where the synthetic oil acts as a thinner oil than the mineral oil equivalent.


[ February 22, 2005, 01:01 PM: Message edited by: AEHaas ]
On a temperature vs viscosity graph the visc. varies linearly with temperature when it is warm. It is logarythmic and shoots through the roof at cold temperatures. If you had one oil with a pour point of -40 and another of -45 the viscosity of the -40 oil could be 10 times more than the -45 oil in the neighbourhood of -39.

This doesn't answer the question you posed but I figured I through it on the table. There is a graph in the Diesel section somewhere.

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