Which of the following is not a normal additive in lubrication?

Hexihydrene is not a normal additive in lubrication because it is not a recognized or commonly used additive in lubricating oils and greases. The standard additives in lubrication typically aim to enhance the performance characteristics of the base oil, such as improving its viscosity, cleanliness, and reducing friction.

Viscosity index improvers are widely used to maintain consistent viscosity across varying temperatures, helping the lubrication perform effectively under different conditions. Detergents are included to keep surfaces clean by preventing deposits and sludge, which is crucial for maintaining engine and machinery efficiency. Friction modifiers work to reduce the friction between surfaces, thereby improving efficiency and minimizing wear.

Hexihydrene, on the other hand, does not serve a recognized role in improving lubrication properties, making it the outlier among the substances listed. Understanding the function of standard lubrication additives is important for maintaining optimal performance in machinery and equipment.