Check color, smell, aeration and water content, and take a sample if you have any doubt. Dark oil that smells burnt has already been overheated; milky oil contains water; foamy oil has air entering somewhere. Each points at a different cause, and all are visible without instruments.
How to confirm it
- Withdraw a sample from a running system
Take oil from a sample point or from the tank with the machine at operating temperature and the oil circulating. Sampling from a cold, settled tank gives you the cleanest oil in the machine rather than a representative sample.
- Look at the color against a clean background
New hydraulic oil is amber and translucent. Oil that has darkened gradually is normal with hours. Oil that is black, or dark with visible suspended material, indicates overheating or contamination and warrants a sample for analysis.
- Smell the sample
Burnt or acrid smell indicates overheating. Hydraulic oil that has run hot loses its additive package and its protective properties, and it will not recover. This is one of the clearest signals available without equipment.
- Check for aeration and foam
Foamy oil or a tank that froths indicates air entering the suction side, a low oil level or degraded anti-foam additives. Aerated oil compresses, so the machine feels spongy and controls lack precision, and it transfers heat badly.
- Check for water
Milky or cloudy appearance means water. Even modest water content accelerates corrosion and additive depletion, and free water at the suction can cavitate the pump. Water usually enters through condensation, a failed cooler or a contaminated fill.
- Wipe a magnet through the sample
Metal particles on the magnet indicate wear somewhere in the circuit. Fine gray fuzz is normal. Flakes or chips are not, and they indicate a component is failing and circulating material through everything else.
- Send a sample for analysis at every oil change on critical machines
Laboratory analysis gives particle counts, water content and wear metal concentrations that visual inspection cannot. On machines where a pump or motor failure would be expensive, the sample is the cheapest insurance available.
What to do about it
Dark or burnt-smelling oil: change it, and find the heat source. Overheating has a cause, usually continuous bypass over a relief or a blocked cooler, and new oil will be degraded the same way if the cause remains.
Milky oil: change the oil, find and fix the water ingress, and change again at a short interval to remove residual moisture. A failed cooler or condensation through a badly sealed breather are the usual routes.
Foamy oil: check oil level, suction hose integrity and suction fittings. Air ingress must be corrected before the oil is changed, or the new oil will aerate as well.
Metal in the oil: identify the source. Open the return filter and examine what it captured, then check the most probable components. Continuing to run with metal circulating damages whatever is still healthy.
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Request a quoteFrequently asked
How often should hydraulic oil be sampled?
On machines where downtime is expensive, at every oil change and after any component replacement. On a single machine under light use, an annual sample plus visual checks at each service is a reasonable compromise.
My oil is dark but the machine works fine. Should I change it?
Dark oil is not automatically failed oil — some darkening is normal with hours. Burnt smell, visible contamination or elevated water content are the findings that demand action. Sampling and analysis turns this from a guess into a decision.
Can I top up with a different brand of hydraulic oil?
Only if the specification matches. Mixing oils of different additive packages can degrade the mixture's properties, particularly anti-foam and anti-wear performance. If it must be topped up in the field, top up to specification and plan a full change.