If the oil is not changed on a routine basis, what will continue to build up until sludge begins to form?

When oil is not changed regularly in an engine, it begins to degrade due to factors such as heat and contamination from combustion byproducts. This degradation leads to the accumulation of various contaminants. Over time, these contaminants combine with the oil, resulting in the formation of sludge, which is a thick, sticky substance.

As the oil ages, it loses its viscosity and lubrication properties, allowing for the buildup of particles, dirt, and debris. These elements contribute to the thickening of the oil, ultimately leading to sludge formation. Sludge can hinder engine performance, reduce oil flow, and cause significant engine damage if not addressed.

In contrast, while metal flakes, nitrates, and water might be present in the oil, the term 'sludge' specifically refers to the buildup of these degradation products when the oil is not changed, making it the most appropriate answer in this context.