What is the reason of viscosity increseing of engine oil?

The viscosity of engine oil increases due to factors such as temperature fluctuations, the presence of contaminants, and the breakdown of oil additives over time. As oil ages, it can oxidize and form sludge, which thickens the oil and reduces its ability to flow easily. Additionally, high operating temperatures can lead to the evaporation of lighter fractions, increasing the oil's viscosity. This thickening can impair lubrication and engine performance, making regular oil changes essential.