Hydraulic fluids are a large group of liquids made of many kinds of chemicals. They are used in automobile automatic transmissions, brakes, and power steering; forklift trucks; tractors; bulldozers; industrial machinery; and aircraft. The three most common types of hydraulic fluids are mineral oil, organophosphate ester, and polyalphaolefin. Others are based on glycol esters and ethers, castor oil, or silicone. Some of the trade names for hydraulic fluids include Durad®, Fyrquel®, Skydrol®, Houghton-Safe®, Pydraul®, Reofos®, Reolube®, Hydraunycoil®, and Quintolubric®.

Brake fluid is a subtype of hydraulic fluid with high boiling point and low freezing point.

Hydraulic systems, like the ones mentioned above, will work efficiently if the hydraulic fluid used has low compressibility.

Some hydraulic fluids have a bland, oily smell and others have no smell; some will burn and some will not burn. Certain hydraulic fluids are produced from crude oil and others are manufactured by many companies.

Hydraulic fluids can contain a wide range of various chemical compounds; oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), phosphate esters (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors, etc.

It was reported that 3,800 patients of the Duke University Health System in Durham and Raleigh were operated on in late 2004 with instruments that were washed in PAG (polyalkylene glycol) hydraulic fluid 32-AW instead of detergent. This occurred after an elevator company drained hydraulic fluid into empty Mon-Klenz detergent barrels, and the detergent supplier redistributed them as detergent; both substances are similar in appearance, similar to maple syrup. The problem was not fixed for weeks in spite of the fact that staff members complained that the tools were "slick"; however a small amount of lubricant is commonly applied to them, as a corrosion protection and to protect the instruments from locking up, therefore it took some time to figure out the oiliness is beyond normal level. The hospital claims that there are no risks associated with the mix-up however, patients have reported lingering health concerns. ([1] hydraulic-fluid-news.php)