Certainly, motor oil is slippery. That's what helps protect an engine's
moving parts. But motor oil does much more than lubricate. It helps cool
the engine, keep it clean, prevent corrosion, and reduce friction to
improve fuel economy. To do all of this, refiners blend in various
additives, which account for 10 to 25 percent of the product you buy.
Some of the additives found in modern oils:
Antioxidants: Prevent the oil from thickening when it runs hot for
Detergents: Help prevent varnish and sludge on engine parts and
neutralizes acid formed in engine.
Pour-Point Depressants: Help the oil flow in a cold engine,
especially in cold weather.
Friction Modifiers: Strengthen the oil films and prevents
unlubricated contact between moving parts.
Viscosity-Index Improvers: Modify the oil so its viscosity is more
consistent over a wide temperature range.
Dispersants: Keep contaminants suspended so they don't form deposits
in the engine.
Rust and Corrosion Inhibitors: Protect metal parts from acids and
water formed in the engine.
Foam Inhibitors: Collapse the bubbles churned up by engine
crankshaft. (Foam reduces lubricating effectiveness.)
Antiwear Agents: Provide lubrication when oil is squeezed from
between moving engine parts.
Additives are consumed in the normal operation of an engine. When used
oil is recycled and then re-refined, it goes through a 14-stage
cleaning process in which additives are integrated, restoring
viscosity to meet the requirements of your automobile. Re-refined oil
is approved by the American
Resource: Consumer Reports July '96