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I have come across two definitions of kinematic viscosity and I am wondering about their relationship and significance in engine oils.
One definition describes kinematic viscosity as momentum diffusivity, while the other defines it as resistance to flow and shear due to gravity (commonly used in lubricants-specific texts). I am curious why there are two different definitions for the same fluid property and whether these definitions are related in any way.

Additionally, I am curious why kinematic viscosity is more significant than dynamic viscosity for engine oils. I have noticed that oil companies only mention kinematic viscosity on oil bottles, such as SAE 5W-30, without mentioning dynamic viscosity. Is dynamic viscosity not useful practically?

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In the daily practice of monitoring the output of a full-scale oil refinery, the process engineers need a simple, repeatable and quick field test for oil viscosity that could be performed right on the factory floor. Many years ago they settled on measuring the time it took for an oil sample to drain out of a standard measuring cup with a calibrated hole drilled in the bottom of it.

The guy who invented the test was named Redwood, and the viscosity unit was named the Redwood-second.

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  • $\begingroup$ That's an interesting piece of information. Is there any practical use of dynamic viscosity? $\endgroup$
    – MechaTrex
    Mar 7, 2023 at 2:49
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Perhaps the general public (excluding those who search for "710" as that is what is written on the cap on top of the engine) have sufficient information to get the correct oil based on all the parties using the SAE ratings, while the extra information is needed by the "power" users who have greater understanding.

Without the actual quotes of the definitions you are referring to it is difficult to differentiate between your understanding or errors by the authors.

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    $\begingroup$ In most fluid mechanics and heat transfer books, kinematic viscosity is described as momentum diffusivity. But in lubricants-specific texts it is defined as resistance to flow and shear due to gravity, along with that method to measure kinematic viscosity is mentioned i.e. capillary test - works by measuring the time it takes for a small volume of fluid to flow through a capillary tube under the influence of gravity. $\endgroup$
    – MechaTrex
    Mar 5, 2023 at 16:17
  • $\begingroup$ @MechaTrex oh books like Mechanics of Machines or Engineering Thermodynamics, Work and Heat Transfer? You might find them a good read - got them in my library... $\endgroup$
    – Solar Mike
    Mar 5, 2023 at 16:29
  • $\begingroup$ Yes, currently I am reading Heat Transfer by Incropera $\endgroup$
    – MechaTrex
    Mar 5, 2023 at 18:31
  • $\begingroup$ Try Engineering Heat Transfer by Simonson. $\endgroup$
    – Solar Mike
    Mar 5, 2023 at 18:36
  • $\begingroup$ @MechaTrex I used to work alongside Incropera's daughter Terri at hewlett-packard in the 1980's. She was SMART. $\endgroup$ Mar 5, 2023 at 20:20

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