Hydraulic oil degrades over time through heat, contamination and additive depletion. A full oil change restores system performance, protects pumps and valves from accelerated wear, and extends the working life of the machine. It is one of the higher-cost maintenance tasks, but far cheaper than replacing a hydraulic pump.
This guide walks through the complete hydraulic oil change procedure, from warming and draining through to bleeding and pressure testing. It applies to most mobile hydraulic systems on excavators, loaders, cranes and similar plant. Always confirm the oil specification, system capacity and filter part numbers in your OEM service manual.
Before you start
Gather your tools and materials: a drain pan sized for the full system capacity, the correct grade of new hydraulic oil, replacement filters, a wrench set, fluid sample bottles, a funnel and PPE (gloves, safety glasses, overalls). Have the maintenance checklist ready to record each step.
Check the OEM manual for the hydraulic oil specification. Common specifications include ISO VG 46, ISO VG 68 and manufacturer-specific formulations such as CAT HYDO Advanced 10 or Komatsu Genuine Hydraulic Oil. Using the wrong viscosity grade can damage seals and reduce system efficiency.
Step-by-step oil change
1. Warm the hydraulic system
Start the engine at idle and cycle all hydraulic functions (boom, arm, bucket, swing, travel) for 10 to 15 minutes. Warm oil flows more freely, carries suspended contaminants out of the system and drains more completely. This step is especially important in cold weather where cold oil can be too viscous to drain fully.
2. Shut down, lock out and depressurise
Turn off the engine and engage lockout/tagout procedures. Cycle the hydraulic controls several times with the engine off to relieve residual pressure in accumulators and cylinders. Remove the reservoir filler cap slowly to release any remaining tank pressure.
3. Drain the old hydraulic oil
Position the drain pan under the reservoir drain plug. Remove the plug and allow the oil to drain completely. On large systems (100+ litres), this can take 15 to 20 minutes. Take a fluid sample from the drain stream for laboratory analysis before the pan collects sediment.
- Also drain the hydraulic cooler if accessible
- Drain accessible hose runs to remove as much old oil as possible
- Inspect the drain plug magnet (if fitted) for metal particles
4. Replace the hydraulic filter
Remove and discard the old return-line and pressure-line filters. Cut open the old filter element with a filter cutter and inspect the media for metal particles, fibre debris or unusual discolouration. This gives you a window into the health of internal components like pumps, motors and valves.
Install new OEM-spec filters. Pre-fill spin-on types with clean hydraulic oil to minimise air introduction on start-up.
5. Refill with new hydraulic oil
Reinstall the drain plug with a new washer if required. Fill the reservoir through the filler neck using a clean funnel. Pour slowly to avoid introducing air. Fill to the correct level on the sight glass or dipstick, accounting for the oil that will fill the lines and cylinders on first operation.
6. Bleed air and test the system
Start the engine at low idle. Slowly cycle each hydraulic function through its full range multiple times to bleed trapped air. Spongy or jerky operation is normal until the air is purged. Monitor the reservoir level and top up as needed. Once operation feels smooth, check all connections for leaks and verify system pressures against OEM specifications with a calibrated gauge.
Oil specifications and selection
| Specification | Viscosity grade | Typical application |
|---|---|---|
| ISO VG 32 | Light | Cold climates, precision hydraulics |
| ISO VG 46 | Medium | Most mobile equipment (temperate climates) |
| ISO VG 68 | Heavy | High-temperature or high-pressure systems |
| Multi-grade (e.g. 10W-30) | Variable | Wide temperature range, some OEM-specific |
Always match the viscosity grade to the OEM specification. Using oil that is too thin increases internal leakage and reduces system pressure. Oil that is too thick increases pump effort, operating temperature and energy consumption.
Contamination signs
Inspect the drained oil before disposal. The condition of the old oil tells you a lot about what is happening inside the system.
| Appearance | Likely cause | Action |
|---|---|---|
| Milky or cloudy | Water contamination | Check seals, breather cap, cooler for leaks |
| Dark brown/black | Heat degradation or oxidation | Check cooler, relief valve settings |
| Metallic particles | Internal component wear | Send oil for analysis, inspect pump and motors |
| Foamy | Air ingress on suction side | Check suction line fittings and hose condition |
| Burnt smell | Severe overheating | Investigate cooler blockage, overloading |
Disposal and compliance
Used hydraulic oil is classified as regulated waste in Australia under state EPA legislation. It must never be poured on the ground, into drains or mixed with general waste. Collect used oil in sealed containers, label them clearly and arrange collection by a licensed waste oil recycler.
Keep records of volumes disposed, dates and the name of the collection company. These records are required during environmental audits and may be requested by your insurer or WHS regulator. A preventive maintenance checklist with a disposal field helps standardise this across your team.
Going digital with MapTrack
Tracking oil change intervals, fluid analysis results and disposal records across a fleet is difficult on paper. With MapTrack, you can scan a QR code on each machine to log the service, record fluid volumes and attach oil analysis reports directly to the asset record.
The maintenance module triggers automated alerts when a machine approaches its next oil change interval based on operating hours. All records are stored centrally, giving your team a complete audit trail for compliance reporting and warranty claims.
