When it comes to hydraulic fluid, it’s not possible to make one, definitive recommendation that covers all types of hydraulic equipment in all applications. But in response to the common question: “What type of oil should I use in my hydraulic machine?”, here are 3 important factors to consider:

Multigrade versus monograde

The operating temperature range of the machine is the factor that determines whether a multigrade or monograde oil is required. If the machine is required to operate in freezing temperatures in winter and tropical conditions in summer, then multigrade oil will likely be required to maintain oil viscosity within optimum limits across this wide operating temperature range.

If the machine has a narrow operating temperature range, and it’s possible to maintain optimum fluid viscosity using monograde oil, I wouldn’t chose a multigrade. Multigrade hydraulic oils are more expensive. And if a monograde oil is used, there is no possibility of loss of viscosity as a result of VI improver shear down.

Detergent versus non-detergent

Detergent oils have the ability to emulsify water, and disperse and suspend other contaminants such as varnish and sludge. This keeps components free from deposits but means that contaminants are not precipitated out – they must be filtered out. These are desirable properties in mobile hydraulic machines which, unlike their industrial counterparts, have limited opportunity for the settling and precipitation of contaminants at the reservoir, due to its smaller volume.

When using an oil with detersive/dispersive additives, it’s even more important to closely monitor water content via oil analysis. Water accelerates aging of the oil, reduces lubricity and filterability, reduces seal life and leads to corrosion and cavitation. And emulsified water can be turned into steam at highly loaded parts of the system. These problems can be largely avoided by maintaining water content below 100 ppm.

Anti-wear versus no anti-wear

The purpose of anti-wear additives is to maintain lubrication under boundary conditions (loss of full-film lubrication). The most common anti-wear additive used in engine and hydraulic oil is Zinc dialkyl dithiophosphate (ZDDP). The presence of ZDDP is not always seen as a positive, due to the fact that it can chemically break down in the presence of water, and attack some yellow metals. However, an anti-wear additive is essential for any oil used in high-pressure, high-performance hydraulic systems