There are different degrees of severity under which boundary lubrication conditions prevail. Some are only moderate, while others are extreme. Boundary conditions are met by a variety of special lubricants with properties corresponding to the severity of the particular application.

These properties are derived from various additives contained in the oil — some singly and some in combination with other additives. Their effect is to increase the load-carrying ability of the oil.

Where loads are only mildly severe, an additive of the class known as oiliness agents or film-strength additives is applicable. Worm-gear and pneumatic-tool lubricants are often fortified with these types of agents.

Where loads are moderately severe, anti-wear agents or mild EP additives are used. These additives are particularly desirable in hydraulic oils and engine oils.

For more heavily loaded parts, a more potent class of additives is required. These are called extreme pressure (EP) agents.

-From the book "Practical Lubrication for Industrial Facilities":