Gearbox Oil Guide: Types, Change Intervals and Common Mistakes
Gearbox Oil Guide: Types, Change Intervals and Common Mistakes
Oil is the cheapest component in a gearbox and the most common cause of premature failure. Over half the field failures we are asked to assess show oil-related damage: wrong viscosity, wrong type, too little, too much, or simply never changed. This guide covers what to use, when to change it, and the mistakes we see most often.
The three oil families
| Type | Typical use | Notes |
|---|---|---|
| Mineral gear oil (CLP) | Standard duty, ambient above 0°C | Cheapest; change most often |
| Synthetic PAO | Wide temperature range, long intervals | Best general-purpose upgrade; good cold start behaviour |
| Synthetic PAG | Worm gearboxes, severe sliding contact | Excellent for worm units; never mix with PAO or mineral |
The PAG warning is not theoretical. Mixing PAG with mineral or PAO oil produces sludge and can destroy a gearbox within days. If you switch a unit to PAG, drain completely, flush, and label the gearbox clearly.
Viscosity: what to pick
Viscosity is set mainly by output speed and ambient temperature:
| Output speed | Ambient | Typical ISO VG |
|---|---|---|
| Above 300 rpm | 0 to 40°C | VG 150 |
| 30–300 rpm | 0 to 40°C | VG 220 |
| Below 30 rpm | 0 to 40°C | VG 320 – 460 |
| Cold start below −10°C | — | synthetic VG 150 – 220 |
Too thin and the oil film breaks down under load. Too thick and the oil cannot reach the bearings at low temperature, so the gearbox runs dry on the first minutes after every cold start — which, over a few years, is enough to destroy it.
Change intervals
| Oil type | Ambient clean | High dust / high humidity |
|---|---|---|
| Mineral | 2,000–3,000 h or 12 months | 1,000–2,000 h or 6 months |
| Synthetic PAO | 8,000–10,000 h or 24 months | 4,000–6,000 h or 12 months |
| Synthetic PAG (worm units) | 4,000–8,000 h | 2,000–4,000 h |
First oil change matters most: on a new gearbox, change after 200–500 running hours, because running-in produces metallic particles that act as grinding paste. Skipping the first change is the most expensive shortcut in maintenance.
The seven most common oil mistakes
- Never changing it at all — the "sealed for life" myth. Very few industrial gearboxes are genuinely lubricated for life.
- Filling to the top — overfilling causes churning, overheating and seal failure. Fill to the oil level plug, never above.
- Using the wrong viscosity because "thicker is better" — it is not, especially in cold weather.
- Mixing oil brands and types — additive packages can be incompatible; PAG and PAO must never mix.
- Ignoring breather condition — a blocked breather pressurises the housing and pushes oil out through the seals.
- Reusing the drain plug washer — cheap part, expensive leak.
- Draining cold and calling it flushed — cold oil leaves the heaviest contamination in the sump. Drain warm.
Reading oil condition without a lab
If laboratory sampling is not practical, do the simple checks:
- Colour — dark but transparent is normal; milky means water ingress; black and gritty means overheating or severe wear.
- Smell — a burnt smell indicates operating temperatures well above design.
- Magnetism — hold a magnet in the drained oil; heavy ferrous sludge means gear or bearing wear is already under way.
- Consistency — stringy or gelled oil indicates contamination or the wrong mixing.
Any of those findings should trigger a gearbox inspection, not just an oil change.
What to write on the maintenance log
For each unit record: date, running hours, oil brand and viscosity, quantity added, drain plug condition, and whether samples were taken. This log turns "the gearbox failed early" from an argument into a date-stamped fact.
Supporting your units
WGT supplies gearboxes with a defined oil specification on the nameplate and in the documentation. If your unit is a WGT gearbox and you need the original oil grade, output speed or mounting position, see our after-sales service page or send the nameplate photograph through the contact page and we will confirm the correct grade and quantity.
Frequently asked questions
Can I use engine oil in a gearbox?
No. Engine oils are formulated for combustion by-products and different additive chemistry. Use an industrial gear oil (CLP) to the correct viscosity grade.
How do I know if there is water in the oil?
Milky or cloudy appearance, and a hiss when the drain plug is released. Water ingress usually means a failed seal, a blocked breather or washdown ingress.
Is synthetic oil worth the extra cost?
Yes for units running continuously, in hot or cold environments, or where access for maintenance is difficult. For light intermittent duty, mineral oil with frequent changes is adequate.
