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Why does hydraulic oil change colour?

It’s completely understandable to be concerned if your hydraulic oil changes from its usual golden colour to a less pleasing shade of dark brown. They worry is that this change could mean something very serious for their machinery, which if not fixed quickly could cause a serious problem.

However, we need to take into account the fact that industrial hydraulic oil is a very different beast from internal combustion oil, simply because it is used in a very different environment to the aforementioned lubricant. So, although noticing a change in the colour of one’s hydraulic oil is certainly a sign to keep a close eye on things, it is not such a serious problem that one must act to replace it straight away. Before considering that, it’s important to first determine the cause of the oil’s colour change.

Common causes of colour changes in hydraulic oil

There are two things that are most likely to cause a change in the colour of hydraulic oil: oxidation and thermal stress, and both of these things do not immediately require an oil change.

The first thing that should be done when a change in oil colour is noted is to take an oil sample and have it analysed by an expert. They will be able to determine whether the oil is still useable, or whether it would need to be replaced, thus potentially saving a significant sum of money.

Hot spots

More of a cause of oil darkening that is a concern, but one which also does not always mean an oil change is needed, are ‘hot spots’ – places in the system where the hydraulic oil is heated up excessively, but which becomes cool again once it reaches the reservoir. An infrared camera can be used to determine whether this is the problem causing oil to change colour, upon which repairs to the equipment can be made.


Whilst mild levels of oxidation are not too concerning, if oil becomes excessively oxidized, its acid levels will increase. This means that machinery components will probably begin to corrode and oil viscosity will get higher, which means that it is important to have it analysed. If it is found to be highly oxidized, it is always a good idea to have it replaced, so that you can have complete confidence in your machinery in the future.

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