The first hydraulic fluid was water - that's right. The Egyptians used water as fluid power. Then sometime in the 1920s mineral oils started being used due to obtain lubrication properties and increase the boiling point of the fluid.
Most hydraulic oils are based on mineral oil base stock.
Brake fluid is a type of hydraulic fluid with a high boiling point, as is power steering fluid.
Choosing one can be challenging. Lots of confusion with users looking for RO, AW, ISO VG, etc.
First things first, figure out what your equipment recommends. If the manual or data plate says AW ISO VG 46, you're looking for an anti-wear hydraulic fluid with a viscosity grade of 46.
Keep in mind that whether you need AW (anti-wear) properties or not is dependent upon your machinery. You'll also find ashless, zinc free, etc. A lot of RO fluids do not contain AW unless they specifically mention otherwise. Knowing what your equipment requires is the best first step.
Next, be sure to determine temperature ranges your fluids will be experiencing. Some hydraulic oils are designed with higher viscosity indexes (VIs) to handle temperature swings better.
There's also fire resistant hydraulic fluids for those really sensitive areas.
And don't forget about Vessel General Permit (US Coast Guard), OSPAR, and Environmentally Acceptable Lubricants. Depending on where you're working and how you're operating, this may be a factor you're unaware of.
Getting down to the nitty gritty. Be sure to keep your hydraulic oil clean, free of water and dirt, and in a lot of cases you should be filtering from the package into your reservoir. Quality hydraulic oil and a filtering process can make the difference in keeping your systems trouble-free.
Hydraulic fluids primarily convey power. Fluid power. Hydraulic systems work best if the hydraulic fluid has zero compressibility.
For use as a power transfer and control medium (most applications) the fluid needs to be non compressible, with a fast air release, a low tendency to foam and low volatility.
For use as medium of heat transfer the fluid should have good thermal capacity and conductivity.
When you're talking about a sealing medium, the fluid needs adequate viscosity and viscosity index (VI) along with shear stability.
As a lubricant, hydraulic oil should have proper viscosity to maintain film protection, low temperature fluidity, thermal and oxidative stability, hydrolytic stability (water tolerance), cleanliness and filterability, demulsibility, antiwear characteristics, and corrosion control.