Base oils are the building blocks of any lubricant.

These are either mineral oils derived from crude oil by careful refining or synthetic base fluids manufactured in a chemical process plant.
 
Base oils can have widely differing physical and chemical properties. This makes it possible for the lubricant blending chemist to formulate a wide range of lubricating oils. Some base oils are so pure that they allow the lubricants manufacturer to make up very long life lubricants for use in turbines. Other base oils are very viscous and allow a thick film formation for lubricating very heavily loaded open gears or large gearboxes.

Base oils derived from crude oil may be classed as either paraffinic or naphthenic depending on their composition. The American Petroleum Institute classes base oils in five groups depending on their sulphur content and their saturate composition.

Base oils termed " Unconventional Base Oils " have appeared in the market. They offer users performance properties very close to those of synthetic base fluids. Synthetic base fluids could be PAO fluids, Polyol Ester fluids, Polyglycol fluids, Phosphate Ester Fire Resistant fluids. All of these specialised fluids have niche roles to play in the design of machinery elements.