Hydraulic oil is the fluid that works a mini excavator’s muscles: the boom rises, the bucket closes, the machine slews and travels — all through the pressure this oil carries. Even so, it isn’t talked about as much as engine oil, and it is often brushed off with the question “is the tank full?”.
Yet oil that is wrong, contaminated or at the end of its life affects the whole system, from the pump to the valves, from the cylinder seals to the travel motors. And these are among the most expensive parts of the machine.
In this guide we explain what hydraulic oil does in the machine, what the codes on the drum mean, the questions to ask when choosing the right oil, when the oil should be changed and how to do the change step by step. Let’s say it up front: your machine’s operator’s manual always has the final word on oil type and change interval; this article is here to make it easier to read and apply.
What does hydraulic oil do in the machine?
Hydraulic oil has four jobs, and each one explains why you need to be careful when choosing it:
- Transmitting power: It carries the pressure produced by the pump to the cylinders and hydraulic motors. For this, the oil needs to be clean and free of foam.
- Lubrication: It forms a thin film between the precision surfaces inside the pump, valves and motors. If viscosity drops too far, this film thins and wear begins.
- Cooling: It carries the heat generated in the system to the tank and the oil cooler. If the oil overheats, it ages faster.
- Sealing and protection: Working together with the seals, it prevents leaks and protects metal surfaces against rust and corrosion.
Anything that weakens one of these jobs — wrong viscosity, water, air or dust, for example — weakens the others too. That is why the oil itself and cleanliness habits are at the root of a large share of hydraulic faults.
Reading the codes on the drum: ISO VG and oil types
On hydraulic oil drums you will most often see two pieces of information: the viscosity grade and the oil type.
The ISO VG number indicates the approximate mid-point of the oil’s kinematic viscosity at 40 °C. The higher the number, the thicker the oil. The grades most often found on construction machines are:
| Grade | What does it tell you? | Where is it usually preferred? |
|---|---|---|
| ISO VG 32 | Thinner oil | Cold climates and work with frequent cold starts |
| ISO VG 46 | Medium viscosity | General-purpose use in temperate climates |
| ISO VG 68 | Thicker oil | Hot climates and long periods of heavy work at high temperatures |
The table shows the general trend; it isn’t a prescription. Your machine’s manual gives either a specific grade or a selection table based on outside temperature; the choice is made according to that table.
The abbreviations showing the oil type matter too:
- HLP or HM: Mineral hydraulic oils with anti-wear additives; the most common group on construction machines.
- HVLP or HV: Oils with a high viscosity index, meaning their consistency changes less as the temperature changes. They may be preferred in regions with a big temperature difference between summer and winter, or for machines working outdoors.
- Biodegradable hydraulic fluids: Used for work near water sources and sensitive areas; they are not mixed with mineral oil, and their suitability for the system must be confirmed in advance.
A frequently asked question is whether you need separate oils for summer and winter. If the grade recommended in the manual covers your region’s temperature range, a seasonal change usually isn’t necessary. In regions with very hot summers and very cold winters, however, an oil with a high viscosity index can help cover the difference between the two seasons with a single oil; make this decision by looking at the table in the manual too.
A warning: some manufacturers may specify a special fluid or a particular engine oil grade for the hydraulic system. That is why the rote rule “hydraulic oil means grade 46” doesn’t fit every machine. Whatever the manual says applies.
5 questions for choosing the right oil
- What does the manual say? The oil type, viscosity grade and, if there is one, the temperature table.
- In what climate and season does the machine work? In a region with harsh winters, a thinner oil or one with a high viscosity index may come into consideration; in a hot region, the upper grade in the manual.
- Do you use an attachment that generates heat? Attachments that run for long periods, such as hydraulic breakers and augers, heat the oil; cooler cleanliness and oil condition should be monitored more often.
- What oil is in the system now? Use the same type and grade for topping up. If you don’t know what’s inside, consider a planned complete change rather than risking a mix.
- How do you store the oil? Even the right oil deteriorates in an open drum, exposed to dust and moisture.
When is it changed? Hours, calendar and warning signs
Hydraulic oil is changed at much longer intervals than engine oil. On compact machines the interval varies across a wide band depending on the manufacturer; many manuals show intervals of around 1,000 hours and above, while shorter intervals may be defined for breaker use. The value in your own machine’s manual applies. We show where hydraulic oil fits among the other maintenance jobs in our service schedule.
Hours aren’t the only measure. In a machine that works little, the oil deteriorates over time by absorbing moisture from the air and oxidising, so keep track of the calendar limit too. The following signs indicate that the oil may need changing early:
- The oil has turned noticeably darker and smells burnt.
- The oil has gone milky; water may have got in.
- There is foam in the sight glass or the tank.
- There are visible particles or sediment in the oil.
- The machine is getting unusually hot, and movements are slowing down or jerky.
If you use your machine heavily, using laboratories that take and analyse oil samples is the clearest way to see the oil’s real condition and the wear in the system. Take the sample in a clean container while the machine is at working temperature; a sample taken in a dusty container gives a false result.
Reading the oil’s condition on site
Even before you send it to a lab, the oil tells you a lot. Look at the sight glass, take a small sample in a clean, clear container and compare it with this table:
| Appearance / sign | Possible cause | What to do? |
|---|---|---|
| Milky and cloudy | Water ingress: condensation, pressure washing, a cap left open | Don’t push the machine; find the source and change the oil and filter |
| Foamy | Air being drawn in, low level, incompatible oil | Top up the level; check the suction line clamps and fittings |
| Dark and smelling burnt | Overheating or oil at the end of its life | Clean the cooler and plan a change; if overheating continues, call service |
| Shiny metal particles | Wear in the pump or hydraulic motor | Contact service without pushing the machine |
| Level keeps dropping | External leak: hose, fitting, cylinder seal | Find the source of the leak; renew the hose or seal |
| Movements slow down noticeably when hot | The oil is overheating or its viscosity isn’t right | Check the cooler and the oil grade; if it persists, call service |
The most common cause of a falling level is external leaks; we cover hoses and connections in our hydraulic hose guide. When the oil cooler fins get clogged with dust and straw, the oil temperature rises; keeping the fins clean directly extends the oil’s life.
Changing hydraulic oil, step by step
Changing hydraulic oil is a job that takes cleanliness and patience. If you are doing it yourself, follow these steps along with the sequence in the manual:
- Preparation: New oil of the type and quantity given in the manual, a new return filter and gaskets, a clean drain pan, a filter funnel or pump, rags and lidded containers for waste oil.
- Put the machine on level ground and lower the attachment to the ground in the position described in the manual.
- Stop the engine. Release the pressure in the lines by moving the control levers as described in the manual, and relieve the tank pressure by opening the tank cap slowly.
- Clean around the tank cap and the filter housing; every opening is a chance for dirt to get in.
- Drain the oil via the drain plug. Warm oil flows better, but hot oil carries a risk of burns; wear gloves and safety glasses.
- Change the return filter; if fitted, clean or replace the suction strainer, and check the breather filter.
- Refit the drain plug with a new gasket and fill the tank with new oil using a filter funnel or pump.
- Run the engine at idle; bleed the air from the system by moving each cylinder slowly several times without bottoming it out.
- Check the level again in the position given in the manual and top up if necessary.
- Check for leaks, clean up any spilt oil and record the change in the maintenance log.
At the end of the first working day after the change, check the level and the filter housing once more. Small air pockets left in the system come out during work and the level may drop slightly; if the filter gasket hasn’t seated properly, a leak will also show itself in the first few hours.
When topping up
A job done as often as changing the oil is topping up the level. Only top up in the measuring position given in the manual and only if the level really is low; an overfilled tank can blow oil out of the breather when the oil heats up and expands. Wipe around the cap before opening it, and use oil of the same type and grade as the oil in the system. Write every top-up in the log with the quantity: if the top-up quantity increases from week to week, there is a growing leak in the system.
Don’t pour waste oil and filters onto the ground or into the drains; hand them over to authorised collection points in line with environmental regulations. If you would like service to do the change, you can open a request using the service request form.
Preventing contamination: the habits that determine oil life
Even new oil quickly turns into dirty oil if it doesn’t enter the system clean. That is why daily habits matter more than the brand of oil:
- Store drums indoors with their caps tightly closed; dust and moisture get into an open drum.
- Don’t carry oil in an open bucket or with a dirty funnel; use a filter funnel or pump.
- Wipe attachment couplings before connecting them and fit the dust caps as soon as you disconnect. Attachments such as breakers and augers share the machine’s oil; one dirty attachment hose contaminates the whole system.
- When changing a hose or part, close off the open ends with a plug or a clean cover.
- When washing the machine, don’t point a pressure washer at the tank cap or breather filter.
- If you work with a breaker for long periods, check the oil temperature and cooler cleanliness more often; we explain the finer points of breaker use in our breaker guide.
In the winter months the oil thickens and the system is strained at cold starts. You will find how to warm up the hydraulics gradually in cold weather in our guide to using a mini excavator in winter.
Summary
- Hydraulic oil transmits power, lubricates, cools and helps sealing; if one of these suffers, they all do.
- The ISO VG number tells you the viscosity, and abbreviations such as HLP or HVLP the oil type; the choice is set by the manual and the climate.
- Don’t mix different types of oil; use the same type and grade for topping up.
- Take the change interval from the manual, but also watch for signs such as cloudiness, foam, darkening and overheating.
- When changing, renew the filter, bleed the air and check the level in the correct position; cleanliness comes first at every step.
For parts such as hydraulic tanks, oil coolers, pumps and valves, take a look at our hydraulic spare parts; for the right oil and filter information for the R350 or any other model you use, ask our service line on WhatsApp.
