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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?

MechaTrex
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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.

niels nielsen
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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.

Solar Mike
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