They are covered with minute peaks and valleys called asperities. When the peaks contact each other, they momentarily weld together, causing friction, heat and wear. Lubricants form a slippery film that prevents the peaks from welding together. It is the load carrying capacity of a lubricant film. Film strength can be enhanced by the use of additives.
Synthetic oils have greater film strength than petroleum oils.
In general, higher viscosity fluids are needed for higher loads and temperatures, lower speeds, rougher finishes and for worm gears. Extreme pressure additives should be used for heavy loads and moderate temperatures, but are not effective with yellow metals such as bronze or brass.