In hydraulic systems, lubricants can sometimes be mixed. Whether done accidentally or not, there are consequences to mixing lubricants. Even mixing the same kind of lubricants can place a system at risk if those lubricants have different viscosities.



How Damage Can Occur

When two lubricants of the same type but differing viscosities are mixed, the lubricant inside the system will be affected. However, to what end it will be affected depends on the amount of each lubricant used as well as the viscosity of each.

Film Thickness Determines Viscosity

The film in any lubricant has one job: to provide separation in a system’s bearings, specifically between the bearing’s stationary and rolling components. Many factors influence the level of thickness needed by a system. These include fluid viscosity, how much load the shaft must bear and how fast the bearing rotates.

If the two mixed fluids do not provide enough in the way of final viscosity, the final thickness of the fluid will be insufficient to provide the separation needed for the components inside the bearing. Left to continue, this inadequate viscosity and separation will lead to failure which may be catastrophic to the system, requiring complete shut down and/or replacement.

The same risks are present if the two mixed fluids provide too much final viscosity. This will result in the final thickness of the fluid being too high, and therefore providing too much separation between the rolling and stationary components in the bearing. This condition will lead to more energy being needed for the hydraulic system to function, and more heat being generated as a result of higher energy use. The effect is the same on bearing components as it would be if a person were running through water; it would become more difficult to run through it as the viscosity increased.


Specific Formulations

There are many different kinds of lubricants on the market. The reason for this is that each type has its own set of properties, formulated specifically so that the fluid can be used in certain types of machine. There are many formulations, but the most common are those with Extreme Pressure (EP) additives, and those with Anti-Wear (AW) additives.

Extreme Pressure Additives

Some lubricants, such as gear oil, often contain EP additives with a sulphur-phosphorous formulation. This formulation can be used in virtually any gear configuration except for the worm gear variety. This is because the material used as part of the worm gear is constructed of bronze or brass, which can be left vulnerable to the formulation of sulphur and phosphorous due to lengthy contact with the gear teeth, which generates intense pressure and heat. When this happens, corrosion is the result.


The anti-wear additives in many kinds of hydraulic fluids do not contain the powerful chemicals of their EP counterparts. Although this will not corrode internal components, seals, or O-rings, it does not offer the level of component protection that EP additives do. This is of particular concern if using hydraulic oils which tend to be far lower in viscosity than most oil used for gears.



Mixing Grease and Oil Can Result in Disaster

The dangers of mixing oil and grease are very real. In order to understand why, the way in which grease oil is manufactured must be considered. Usually, the first step in grease oil manufacturing is to first make the thickener, and then add oil until the target grade has been reached.

Should oil be added to grease, the ratio between the thickener and base oil will change. This will result in a grade with lower consistency, but when combined with all of the other factors like viscosity, can cause very serious issues within a system.

Although it may seem easier to mix lubricants, the best thing to do is ensure the same lubricant is always being used in a system, or that the system is flushed so that a new type of lubricant can be used.