Power steering fluid amplifies through hydraulic pressure the force you exert on the steering wheel. Without regular replacement of the fluid, your car's steering will start to groan, then to resist. By the time this problem becomes severe, the pumps that circulate the fluid most likely will be damaged. There are many brands of fluid, each with its own chemical composition, but it is possible to classify them based on manufacturer and intended use.
Dedicated Steering Fluids
The chemical composition of dedicated power steering fluids varies by maker, but all will be marked as "Power Steering Fluids" and usually be composed of mineral oil, polyalphaolefin or organophosphate ester. They may also have glycols, silicons and corrosion inhibitors added.
Not all cars use dedicated power-steering fluid. Some systems use transmission fluid for hydraulic pressure in the steering system. The car's manual should specify which type of transmission fluid should be used in the power steering system.
Manufacturer's Steering Fluids
Many manufacturers, including GM and Honda, make their own steering fluids. Although any kind of power steering fluid will usually work, performance is often optimised by using the manufacturer's label, which can be bought from any parts supplier for that make of car.
Interchangeable Steering Fluids
There are many brands of power steering fluid designed for use in all steering systems. Though they are not distinguished by type, their chemical compositions are different from manufacturer to manufacturer.