Fluid Friction:
Fluid friction manifests when the lubricating fluid is introduced between contact surfaces of the two bodies.
If thickness of lubricant or oil between the mating surfaces is less, then the friction between the surfaces is called as GREASY OR NON-VISCOUS FRICTION. The surfaces absorb oil and the contact between them is no more metal-to-metal contact. Instead contact is through thin layer of oil and which ultimately results are less friction.
When the thick film of lubricant separates two surfaces, metallic contact is totally non-existent. The friction is because of viscosity of the oil, or shear resistance between the layers of oil rubbing against each other. Certainly then these gives a great reduction in friction. This frictional force is also called as Viscous Or Fluid Friction.