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.
Viscosity index improvers
This is not to say that mutli-grade engine oils are suitable for every hydraulic system. Multi-grade engine oil, automatic transmission fluid and anti-wear, high VI (AWH) hydraulic fluid are commonly used in hydraulic systems that experience a wide operating temperature range. These fluids have a higher Viscosity Index (VI) than standard anti-wear hydraulic fluids due to the addition of VI improvers. The higher the VI a fluid has the smaller the variation in viscosity as temperature changes.
In simple terms, this means that if you are running a 10W-40 engine oil in your excavator hydraulics, you can operate the hydraulics with a higher fluid temperature before viscosity falls below optimum, than you could if you were running a single-grade, anti-wear, hydraulic fluid.
When selecting a high VI fluid, it is wise to increase the hydraulic component manufacturer's minimum permissible viscosity value by 30% to compensate for possible loss of viscosity as a result of VI improver sheardown. VI improvers can also have a negative effect on the air-separation properties of the fluid and for this reason some hydraulic component manufacturers recommend that high VI fluids only be used when operating conditions demand.
Conclusion
As far as hydraulic oil recommendations go, for commercial reasons relating to warranty etc, it is always advisable to follow the machine manufacturer's recommendation. But on the other hand, if you were to realize retrospectively that the hydraulic system of your excavator or dozer had been inadvertently charged with a multi-grade, diesel-engine oil, it's not necessary to press the panic button either. Discussing the application with a technical specialist from the oil manufacturer, in conjunction with the OEM, would however be advisable.
