Additive depletion is one of the ways hydraulic oil gets 'spent' and in doing so, necessitates an oil change. Other than keeping the oil (relatively) cool and dry, there's not a lot we can do about additive consumption. But losing additives through over zealous filtration is another matter entirely. Consider this question from one of our members:

"If I filter hydraulic oil with a kidney-loop unit down to half a micron, will I lose some of the additives?"

Before I attempt to answer this question, is filtering hydraulic oil down to 0.5 microns sensible or even warranted? For 99.9 percent of hydraulic systems out there, probably not.

Even for critical, servo-valve systems, filtration of 2-micron absolute is considered adequate. So for the vast majority of systems, you have to question the benefits of sub-micron filtration - particularly when you take into account the COST of doing it. Just because sub-micron filtration technology is available - it doesn't automatically mean its use is justified on a cost/benefit basis.

But can oil additives be removed by fine filtration? It depends-on the filter or filtration process being used and the type of additive.

The first thing to keep in mind is most of the additives in hydraulic and lubricating oils should be dissolved in the base oil. This means they should NOT be in suspension in the oil and therefore, should NOT be captured by conventional depth filters - even fine ones.

There is at least one notable exception though. De-foamant additives are suspended rather than dissolved in the oil and so they can be filtered out. As can any other additives that are not properly dissolved in the oil for any reason.

But conventional depth filters aren't the only form of fine filtration available. It's generally accepted that absorbent type filters which consist of chemically active materials such as activated alumina, fullers earth, active clays, charcoal and chemically treated paper can remove additives from the oil.

Some references assert there is a risk of additive removal with centrifugation, while a research paper I read recently concluded that centrifugal filters installed on diesel engines do not remove soluble additives from the oil.

As you can see, this issue, like many others in hydraulics, is not black and white. But as I mentioned at the beginning, any risk of additive removal can be avoided by not filtering the oil beyond the level justified by the application. And in the vast majority of hydraulic applications where the installed level of filtration is 5 microns or above, the risk of additive removal is virtually non-existent.