Base oil is the main component of lubricants such as engine oils, transmission oils or greases. The type and quantity of base oils used determine the lubricating properties of the lubricant. Properties such as viscosity-temperature behaviour, oxidation resistance or friction behaviour are significantly influenced. Additives can be added to the lubricant to give it even more properties.

Base oils define the inherent properties of the finished product, including viscosity, stability/longevity, and load-bearing capacity (the ability to withstand extreme pressures).

For example, a lubricant made with a high-quality, highly refined base oil may have better thermal and oxidative stability than a lubricant made with a lower-quality base oil but NOT have the same degree of load-bearing capacity as a result of the molecular structure and purity.