Oil additives also have a limited lifetime. Some are consumed as oil ages. For example, alkaline additives get used up by neutralizing corrosive acids produced by the combustion process. When the oils reserve alkalinity (TBN) falls below the minimum safe level, higher component wear can be expected.
Make-up oils will increase oil reserve alkalinity only to the extent of the new oil added and has no neutralizing reaction on existing oil acid levels.
Rust and corrosion inhibitors, anti-oxidants and film strength agents also reach a point when they can no longer carry on. Additive "dispersants" suspend contaminants, deposits and other combustion insoluables until they are removed from the system by oil and filter changeout.
Once a dispersant becomes "loaded" any added sludge, resin or soot will cause the oil to dump whatever it has collected... and refuse to collect anymore. This results in a rapid build-up of engine deposits.
All engines, transmissions and drive-axle component oils oxidize. A chemical reaction between oil molecules and oxygen takes place at high operating temperatures. This reaction increases viscosity, causes formation of insoluble engine deposits and corrosive acids which further increases component wear.
Higher operating temperatures, fuel consumption, rapid additive depletion and substantial loss of power can also be expected when oil oxidation takes place. When severe, oxidation makes the oil very hard to pump causing lubrication starvation to moving parts, with inevitable results. Oils that are oxidized have a very pungent, sour odor.