DIN 51524; HLPD fluids are a class of hydraulic fluids which contain detersive and dispersive additives. The use of these fluids is approved by most major hydraulic component manufacturers and can be advantageous in many applications, including mobile, to prevent build-up of sludge and varnish deposits, which can lead to valve stiction and other reliability problems. The main caution with these fluids is that they have excellent water emulsifying ability, which means that if present, water is not separated out of the fluid. Emulsified water reduces lubricity and filterability, can cause corrosion and cavitation and reduces the life of the oil. These problems can be avoided by maintaining water content below 0.1% - which is not a low water content target for any high-performance hydraulic system. A hydraulic fluid that has the ability to emulsify small amounts of water can be beneficial in mobile applications.
Modern lube chemistry means that detersive and air-separation/anti-foaming properties aren't necessarily mutually exclusive. At least one major, mobile equipment manufacturer specifies heavy-duty diesel engine oil to API classifications CD, CF-4, CG-4 or CH-4 as an acceptable alternative to a single-grade, anti-wear, hydraulic oil in their equipment.
