No two hydraulic oils are the same! Types of hydraulic oil and whether they mix

Users of hydraulic oils need to be aware that no two oils are the same. Practical experience shows that hydraulic oils from different manufacturers can differ quite significantly, even if they meet the same specifications.

If hydraulic fluids from different manufacturers or different types from the same manufacturer are mixed, this can in practice lead to sludge formation, agglutination, deposits due to reactions to additives or even a reduced air release capacity. Under certain circumstances, this can lead to foaming or an increased risk of cavitation. Malfunctions and damage to the hydraulic system are inevitable. This is why many manufacturers of hydraulic systems generally prohibit any mixing with other hydraulic fluids, which also includes hydraulic fluids of the same classification, because modern lubricants are made from a variety of base oils and “ingredients”, i.e. complex combinations of active ingredients (additives). This cocktail of additives can enhance the existing properties of the base oil or give the end product a property required for a specific application. The list of different base oils and additive components is very long and, as experience shows, hydraulic oil manufacturers don't always provide the relevant information.
The basic rule is that just because fluids can be mixed doesn’t automatically mean they are compatible!

The experience of independent oil laboratories in their daily work on hydraulic problems indicates time and again that hydraulic oils with the same base oil, similar viscosity and comparable density may mix very well with each other, but the different additives cause incompatibilities during operation, which then leads to hydraulic faults in the system. 

Please note: what can be chemically mixed can have a highly negative impact on the physical properties of the pressure fluid.

Caution! In their technical data sheets, some oil suppliers claim that their products can be mixed, but this is the only property they confirm. The question that users should ask their oil suppliers is: “Are the hydraulic oils compatible with each other once they have been mixed?”

Mixability describes a chemical property and is only present when two liquids can be easily and completely dissolved in each other.

Compatibility, on the other hand, means that two fluids or two hydraulic oils can be easily mixed with each other, while at the same time retaining their individual properties. The different additive packages must not react with each other. This will happen if, before mixing hydraulic oils with different designations, it is not clarified with the relevant oil supplier whether, in addition to the same base oil properties, the same or similar additives are used. Mixing oils with different base oils and additives should be avoided wherever possible. However, information about the compatibility of lubricants is often difficult to come by from oil suppliers, if at all.

The following hydraulic fluids should never be mixed with each other:

• Hydraulic oils with different classifications (HLP/HLPD/HVLP)
• Hydraulic oils which contain zinc and ones which don't
• Hydraulic oils and motor oils
• Oils with detergent and non-detergent properties
• Oils of the same classification, same manufacturer but with different viscosities
• Biodegradable oils (HEES/HEPG/HETG/HEPR) and mineral oils
• Biodegradable oils of different brands despite the same classification (e.g. HEES)
• Biodegradable oils of different classifications (e.g. HEES with HETG)
• Glycol-based oils with all other synthetic or mineral oils

The following problems can arise:

• Increased risk of surface foaming
• Degradation of air release properties
• Increased risk of cavitation
• Change in behaviour towards water
• Increased wear on seals and hydraulic hose lines
• Change in friction characteristics
• Reduced hydraulic oil service life
• Reduced filterability or reduced filter service life
• Increased system contamination due to dissolved deposits
• Increased formation of sludge, deposits / varnish due to additive reactions
• Clogging, e.g. due to zinc soap

Topping up with fresh oil should only be carried out if the existing oil quality in the system still meets requirements (viscosity, acid content, oxidation). Please note that fresh oil should only be added using filling pumps with the appropriate filter fineness (absolute filtration). Otherwise, there is a risk of dirt entering the system.

Avoid mixing oils, as there is no combination which is guaranteed to be 100% problem-free.

Caution! Depending on the type of fluid and application, incompatibilities can cause rapid and significant hydraulic faults. However, these faults may only become apparent indirectly or in the long term.

Reputable manufacturers of hydraulic machinery or equipment do not permit oil mixtures, and point out that failure to observe this will result in the exclusion of any warranty claims.

Measures to avoid mixing of fluids

• Introduction of a fluid management system
• Careful incoming goods inspection, as the different types of lubricant from the same oil manufacturer differ only slightly in their long columns of letters and numbers.
• A well-organised oil store with clearly marked storage locations for the various lubricants used reduces the risk of confusion.
• An oil filling pump that can be used for different types of lubricants, emptied or flushed after use and only used with filter elements that are specifically marked for the relevant type of oil.
• Training and awareness-raising among maintenance and servicing personnel on the subject of the practical use of hydraulic oils.

Yes, hydraulic fluids can in general be mixed with each other – with the exception of PAG-based hydraulic fluids. However, whether the oils for which a declaration of mixability is provided by the manufacturer are in fact compatible with each other can either be confirmed by the technical support of the oil supplier (and hopefully in writing) or, if not, can only be determined by a detailed and elaborate compatibility analysis in the laboratory. In addition to the additives and the base oil, properties such as air and water release, foaming behaviour, acid content and filterability at different mixing ratios must also be taken into account.

The experience of diagnostic engineers from independent oil laboratories, based on the analysis of thousands of oil samples, shows that the mixing or contamination of a lubricant with the oil of another manufacturer or with a different oil type is one of the main causes of equipment malfunctions and costly breakdowns.

Please note: hydraulic oil is not only a design element that must be carefully considered during the planning stage, but also a working medium that needs to be taken into account during maintenance and servicing, and also requires the appropriate expertise.