The study of lubrication and wear mechanisms is called tribology.
The lubricity of a substance is not a material property, and cannot be measured directly.
This is often done by determining how much wear is caused to a surface by a given wear-inducing object in a given amount of time.
For two fluids with the same viscosity, the one that results in a smaller wear scar is considered to have higher lubricity.
Reformulated diesel fuel that does not have biodiesel added has a lower lubricity and requires lubricity improving additives to prevent excessive engine wear.