The enemies of engine Oil in a 4-stroke engine are heat, additive depletion and contamination.

Heat Accelerates Oxidation and Volitility- The result is thickening of the oil as well as a reduction in total volume. This causes the engine to be harder to start (shorter battery life) and the oil is more difficult to pump (wasted power). The net result is reduced performance and potential metal to metal engine damage if the oil is not topped up and/or replaced.

Some of the Additives found in 4 Stroke Oils are designed to be consumed during the oil’s life. Typically these additives are used up in the process of neutralizing undesirables or by plating out and wearing away.

Contamination is also Normal- 
The combustion process forms a number of compounds, which move past the rings or valve seals and mix with the oil. These compounds when mixed with the oil ultimately make it unfit for use. The only way to remove these contaminates is to change the oil.

Why change your 4-Stroke Oil? 

Additives get used up or depleted over time, placing your engine at risk:

• Dispersants become saturated, and can no longer hold additional soot in suspension, resulting in sludge deposits
• Overbased detergents get used up and can no longer neutralize acidic by-products of combustion, causing acidic corrosion of internal engine parts
• Antiwear additives get depleted, allowing increased engine wear, particularly on the cams and liners
• Viscosity modifiers get chewed up, causing oil to lose its high temperature viscosity control 
• Antioxidants get consumed, causing oil to become extremely viscous and difficult to pump