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The standard answer is to refer to the vehicle owner’s manual. This is not very helpful as the vehicle manual will usually state the manufacturers own fluid.

The problem is that there is no independent standard for power steering fluid so the manufacturers creates their own specification – the result is hundreds of OEM branded fluids and specifications, many of which are virtually identical.

Power steering fluid is made up of vegetable oils, minerals oils or synthetic mineral oils with additives to reduce friction, provide corrosion protection, increase the operating temperature range and increase the operating pressure range.

In our experience power steering fluids can be broadly divided into 3 groups; Automatic Transmission Fluids (ATF); Liquide Hydraulique Mineral (LHM) and Synthetic High Performance (Central) Hydraulic Fluids (CHF). THEY SHOULD NOT BE MIXED and the correct type of fluid should be used in each application.

Automatic Transmission Fluids (ATF)

These are generally coloured red and have been primarily developed for automatic gearboxes. Over the years this has been the fluid most commonly used by the volume car manufacturers. The changes in specification over the years have been mainly to maintain the performance of the fluid over an extended lifetime of operation. 

Higher specification fluid can be used on older models and mixed with lower specification fluids – it just usually costs more with no noticeable benefits.

So if you only want to stock one ATF fluid best to go for a high specification fluid such as a Synthetic Universal Automatic Transmission Fluid that is completely compatible with other synthetic and petroleum automatic transmission fluids. In our experience you will be alright to use it in all makes of vehicles with the exception of Porsche and Audi up to 2002.

Central Hydraulic Fluid (CHF)

CHF is light straw colour or light green and is especially designed to withstand high temperatures and pressures over a long period (100,000 miles). They are used in power steering systems, level control/stability control suspension systems, traction control systems, convertible top hydraulics and central locking systems. 

Many vehicles manufactured after 2005 use CHF and all vehicles fitted with EHPS pumps use CHF in their power steering systems. 

Because they can withstand higher temperatures vehicle manufacturers can use smaller PAS reservoirs, reducing weight, space and “environmental cost of disposal of waste fluid”. USING ATF IN THESE SYSTEMS CAN CAUSE PROBLEMS. ATF will deteriorate when operating at the temperatures experienced in systems designed to use CHF. So if the original fluid is not red, the vehicle is not an old Citroen then use CHF or the vehicle manufacturers recommended fluid.

Liquide Hydraulique Mineral(LHM)

LHM is green and is a mineral based hydraulic fluid specially developed for use in Citroen cars. Citroen developed their suspension, braking and steering system to use a common hydraulic pump which delivered pressure to the suspension system at around 2250 psi. (most power steering systems operate at a maximum pressure of 1250 – 1500 psi). The fluid was developed to cope with these high pressures. If it’s a Citroen Xantia, XM, CX or BX use LHM, if it’s not don’t use LHM.