Why Motor Oil Deteriorates

It is common knowledge that, at some point, engine oil must be
changed. It's something that is preached relentlessly to vehicle
owners by vehicle manufactures, quick lubes and oil companies. But
consumers are widely unaware or what exactly makes oil necessary.
Many factors contribute to a motor oil's demise, but it is
essentially the accumulation of contaminants in the oil and chemical
changes in the oil itself that makes a motor oil unfit for further
service. With time, it is inevitable that the oil will be
contaminated by dirt or sludge, or succumb to the extreme pressure or
temperatures found inside an engine.

One of the reasons:
Extreme Heat
Today's engines are running hotter than ever. More horsepower, turbo
chargers and aerodynamic styling have created extremely hot
environments that receive less cooling from outside air. High heat
leads to oil oxidation, deposits and thickening in conventional oils.
Because they are made from impure, irregular molecules, conventional
motor oils are more susceptible to the effects of heat. The small,
light molecules in conventional oil tend to evaporate as the oil is
heated, leaving large, heavy molecules are chemically unstable, they
may break-down and form deposits on components surfaces, further
inhibiting the release of heat into the oil stream.
Even in relatively mild temperatures, oxygen works to break down some
of the chemicals in conventional lubricants. The extreme heat in
engines actually promotes oxidation. When conventional oil
contaminants break down, they coat components with varnish, deposits
and sludge and leave the lubricant think, hard to pump and with very
poor heat transfer ability.
Look forward to more on why motor deteriorates.