Why is oil used in hydraulic systems

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Although lubricants can usually be mixed well with one another (with the exception of PAGs) - especially if they have the same nominal viscosity and are designed for the same purpose - they are not always compatible with one another. In particular, when zinc-free and zinc-containing HLP mineral oils, PAO or ester synthetic oils are mixed with mineral oils or with synthetic oils from other manufacturers, essential properties such as air or water separation, foam behavior, filterability or compatibility with seals can be impaired. Mixing different hydraulic fluids cannot always be avoided, as is the case with construction machinery, for example, when different attachments are exchanged on a daily basis. If malfunctions occur after the dismantling or after an oil change, the question of the compatibility of two hydraulic fluids often arises. OELCHECK tribologists can more easily assess the cause if they have the following information:

  • Exact oil names, if possible with details of the base oil types and additives
  • Working temperatures and pressures
  • Machine designation and the components used
  • Operating times of the liquids
  • Capacity


In most cases, an indication of the cause of the problems can already be given via a standard analysis, which is carried out using an analysis set 2 or 4. But sometimes it is necessary to conduct an explicit impact assessment. To do this, the laboratory needs five samples (1 liter each): a fresh oil sample each of the two hydraulic fluids used and three mixtures in a ratio of 50:50, 95: 5, 5:95. "Compatibility test" must be noted on the sample information sheet. In the OELCHECK laboratory, tests are initially carried out as for analysis set 2. In addition, the air release capacity, the foam behavior, the water release capacity and the water content are determined. The experienced OELCHECK tribologists then assess whether the oils are compatible with one another and what problems are to be expected.