Which of the following is NOT a factor that causes an increase in viscosity of oil?

Viscosity refers to the thickness or resistance to flow of a fluid, and it can be influenced by several factors including temperature, composition, and the presence of contaminants. Among the choices listed, additives are specifically designed to enhance the performance of lubricating oils, not to increase viscosity significantly.

Additives can serve various purposes, such as improving detergent properties, providing anti-wear protection, or enhancing oxidation stability, but they typically do not cause a significant increase in viscosity. In fact, some additives are formulated to help maintain or lower viscosity, particularly in response to temperature changes.

In contrast, water, air, and soot can all negatively impact the viscosity of oil. Water can cause emulsification, increasing the effective viscosity. Air can lead to oxidation issues and may also contribute to foaming, which alters flow characteristics. Soot, resulting from combustion processes, can accumulate in oil, increasing its viscosity due to the presence of particulates that impede flow.

Thus, the role of additives as enhancers rather than thickening agents is why this choice does not contribute to an increase in viscosity in oil.