What is oiliness?

What is oiliness?
To explain roughly, oiliness refers to the action of sticking to lubricated surfaces, such as metal surfaces, and forming an oil film.
Specifically, oiliness refers to the physical sticking to metal surfaces due to the viscosity and stickiness of oil, or the chemical adsorption of oil to metal surfaces to form an oil film.
An oiliness enhancer is an ingredient that improves these two things, but those that only act physically are not often called oiliness enhancers.
For example, additives that increase viscosity can be called oiliness enhancers in terms of function, but they are only called thickeners, so most of them are called by other names even if they actually improve oiliness. (Postscript: It seems that an additive like MoDTC, which forms a compound film on a metal surface to reduce friction resistance, is also called an oiliness enhancer.)
The component with chemically adsorbing action has an unstable part in its molecule, and that unstable part bonds with the metal surface, and the component and the metal surface are in a state of compounding and sticking to each other.
Lubricants with such an effect include esters, but pure mineral oil and PAO do not have such an effect. In the case of mineral oil, it is easy to deteriorate due to impurities such as sulfur.
I have digressed a little, but I would like to continue the discussion on oiliness in another installment.

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