In addition to sulfur, what substance may enter the engine oil and form sulfuric acid?

The correct answer is water from the cooling system or condensation. When water enters the engine oil, either through leaks from the cooling system or through condensation that occurs during temperature fluctuations, a reaction can take place in the presence of sulfur. This reaction leads to the formation of sulfuric acid, which can be detrimental to the engine components, causing corrosion and degradation of the oil.

Water is particularly relevant because it can mix with sulfur compounds that may already be present in the oil, facilitating the chemical reaction that produces sulfuric acid. This is a significant issue in engine maintenance, as the presence of moisture in the oil can lead to increased wear and decreased lubrication effectiveness.

Other substances listed, such as oxygen from the air, exhaust gases, or fuel additives, do not directly interact with sulfur in the same way to specifically produce sulfuric acid in the engine oil. While oxygen and exhaust gases can contribute to engine wear through different mechanisms, they are not involved in the formation of sulfuric acid from sulfur in the same direct manner as water.