Viscosity is a crucial property of base oils, representing their resistance to flow. It is typically measured at different temperatures. Common viscosity grades include ISO VG (International Standards Organization Viscosity Grade) and SAE (Society of Automotive Engineers) viscosity grades.

This index measures the rate of change of viscosity with temperature. A higher VI indicates better viscosity stability over a range of temperatures.

The flash point is the temperature at which a base oil gives off vapors that can ignite in the presence of an open flame. A higher flash point is generally desirable for safety reasons.

The pour point is the lowest temperature at which a base oil remains fluid. It is a critical factor in cold-weather performance, especially for applications in regions with low temperatures.

Low sulfur content is often preferred to reduce the formation of harmful by-products during combustion and to comply with environmental regulations.

This property measures the resistance of base oils to oxidation, which can lead to the formation of sludge and varnish. High oxidation stability ensures a longer service life for lubricants.

The acid number indicates the level of acidic compounds present in the base oil. Low acid numbers are desirable to prevent corrosion and degradation of lubricant additives.

Base oils are categorized into different groups (Group I, II, III, IV, and V) based on their refining processes and properties. Group I oils are typically solvent-refined, while Group II and III oils undergo more advanced refining processes, resulting in higher-quality base oils.

Base oils should be compatible with various additives to enhance the performance of lubricants. Additives may include anti-wear agents, detergents, dispersants, and friction modifiers.

Low levels of hydrogen sulfide are preferred to reduce the risk of corrosion in certain applications.