Hydraulic oil, or hydraulic fluid, is available in many varieties with differing chemistries. Their densities range from 0.8 grams per milliliter (g/ml) up to about 1.0 g/ml.

The density of a material is the ratio of its mass to the volume of space it occupies. In chemistry and physics, it is usually expressed as grams per milliliter (g/ml). In some fields it may be expressed as pounds per gallon.

Most hydraulic fluids fall into one of three broad categories: mineral oils, polyalkylene glycols (PAGs), or polyalphaolefins (PAOs).

The base stocks for mineral-oil-based fluids are manufactured from petroleum. Mineral oils are therefore hydrocarbons (they contain only carbon and hydrogen). Examples include most tractor fluids and many automotive transmission fluids. These fluids typically exhibit densities on the order of 0.8 to 0.9 g/ml and will float on water.

PAGs are synthetic fluids (not made from petroleum). They are commonly used as automotive brake fluids and as lubricants for air-conditioner compressors. Their densities are typically around 1.0 g/ml.

PAOs are synthetic hydrocarbons that are chemically very similar to mineral oils, but with better lubricating properties at very low and very high temperatures. Like mineral-oil-based fluids, they have a density of 0.8 to 0.9 g/ml.