by Robert Sanborn on Tue, Jun 26, 2007, who was a member of the STLE Society of Tribologists and Lubrication Engineers for 15 years.
Excessive engine heat causes oil oxidation, which in turn results in permanent thickening of the oil. Oxidation products can attack some bearing metals. This was a common problem in engines until research produced a chemical compound capable of interrupting or slowing down the rate of oil oxidation. It was discovered that several different oil-soluble chemicals would accomplish this. Some slowed down the high-temperature oil deterioration process and were called oxidation inhibitors. Others formed a protective coating on sensitive bearing metals, and these were termed bearing corrosion inhibitors. Higher speeds, hotter temperatures, the expanded use of turbochargers, and the widespread use of copper lead bearings require all high-quality motor oils today to contain adequate amounts of oxidation and bearing corrosion inhibitors. Like most additives these are used up in service and must be replenished through regular oil changes.
Motor Oil Guide, American Petroleum Institute