The lubrication should provide a load-bearing film of lubricant, reduce friction in gliding parts, redirect warmth to the oil sump, hold impurities in suspension, seal, protect polished metal parts against corrosion and, in addition, reduce the level of mechanical noise..
The picture below shows an oil pump from the middle of the last century. At that time there was only one sort of oil available and one viscosity for summer and another for the winter period. Instead of an electric pump there was a hand crank.
This doesn't generally occur in the sliding bearings of the vehicle, because either the bearings are installed with lubricant or else oil residue is still present. Nevertheless, in this type of friction, the forces are the greatest.
This is the situation between dry- und liquid friction. Here the sliding friction surfaces are seperated by an oil film, but there are still points of contact. Mixed friction can occur under unfavourable conditions like e.g., high stress and hot oil which has taken on a low viscosity.
This is the normal condition of the sliding bearings. Liquidity seperates the two friction surfaces by hydrostatics (higher oil-pressure) or by hydrodynamics (adapted RPM). 11/11