If you are asking "what is the viscosity of vacuum pump oil?", here is the short answer: most vacuum pump oils are graded between ISO VG 22 and ISO VG 150, which means a kinematic viscosity of roughly 22 to 150 mm²/s (cSt) measured at 40 °C. For the majority of oil-sealed rotary vane pumps, ISO VG 68 (about 68 cSt at 40 °C) is the most widely used grade. Below, we explain what that number really means, why it matters, and how to choose the right one for your pump.
What Does "Viscosity" Mean for Vacuum Pump Oil?
Viscosity describes how resistant a fluid is to flow — in simple terms, how "thick" or "thin" the oil is. For vacuum pump oil, viscosity is expressed as kinematic viscosity, measured in mm²/s (identical to centistokes, cSt) according to the ASTM D445 test method, normally at 40 °C and sometimes also at 100 °C.
To make selection easier, oils are grouped into ISO viscosity grades (ISO VG) under the ISO 3448 standard. Each grade is named by its midpoint viscosity at 40 °C with a tolerance of ±10%. An ISO VG 68 oil, for example, must fall between 61.2 and 74.8 cSt at 40 °C. You may also see a viscosity index (VI) on the data sheet: the higher the VI, the less the viscosity changes as temperature rises or falls — a valuable property for pumps that start cold and run hot.
Typical Viscosity Grades of Vacuum Pump Oil
The table below shows the common ISO VG grades used in vacuum service and where each one is typically applied.
| ISO VG Grade | Kinematic Viscosity at 40 °C (cSt) | Typical Application |
|---|---|---|
| ISO VG 22 | 19.8 – 24.2 | Small pumps, cold climates, easy low-temperature start-up |
| ISO VG 32 | 28.8 – 35.2 | Small rotary vane pumps, unheated workshops |
| ISO VG 46 | 41.4 – 50.6 | Medium-sized pumps in moderate conditions |
| ISO VG 68 | 61.2 – 74.8 | The standard choice for most oil-sealed rotary vane vacuum pumps |
| ISO VG 100 | 90 – 110 | Larger pumps, higher operating temperatures and loads |
| ISO VG 150 | 135 – 165 | Heavy-duty, continuously running pumps in hot environments |
Real products confirm this spread: a general-purpose mineral vacuum pump oil such as Mobil Vacuum Pump Oil is rated at 68.5 cSt at 40 °C, while a low-viscosity product like Busch VM 022 is rated at 22 cSt at 40 °C — both sit squarely inside their ISO VG 68 and ISO VG 22 bands.
Why Viscosity Matters in a Vacuum Pump
In an oil sealed rotary vane vacuum pump, the oil does three jobs at once — it lubricates the moving parts, carries heat away, and seals the tiny clearances between the vanes, rotor, and stator. Viscosity directly controls all three:
- Sealing and ultimate vacuum. The oil film fills the vane-to-stator gap and prevents air from leaking back. Oil that is too thin cannot maintain that seal, so the pump struggles to reach its rated ultimate vacuum.
- Lubrication and wear. A sufficiently thick film keeps metal surfaces apart. Too thin, and vanes and bearings wear early; too thick, and the oil may not circulate fast enough to protect contact points at start-up.
- Starting torque and energy use. Oil that is too thick — especially in a cold workshop — makes the pump hard to start, raises current draw, and can cause cavitation, noise, and overheating.
- Temperature behavior. Viscosity drops as the oil heats up. A grade chosen for cold start-up must still be thick enough to seal and lubricate once the pump reaches its full operating temperature.
How to Choose the Right Viscosity
Choosing the correct rotary vane vacuum pump oil type comes down to four practical questions:
- What does the pump manual say? The manufacturer's recommended ISO VG grade is always the starting point — clearances and oil circuits are designed around it.
- How cold is the start-up environment? In cold conditions, lean toward a lower grade (ISO VG 22–46) or an oil with a high viscosity index so the pump can start and circulate oil quickly.
- How hot and how hard does the pump run? Continuous duty, high ambient temperatures, and large pumps favor ISO VG 100–150 for stronger film strength at temperature.
- What gas and contamination load does it handle? Moisture, solvents, and process vapors degrade oil and thin it out. In harsh service, pair the right grade with a high-quality, oxidation-stable oil and shorten the change interval.
Signs Your Oil Viscosity Is No Longer Right
Even the correct grade stops performing once the oil ages. Watch for these warning signs:
- Oil that looks milky (water contamination), dark, or sludgy
- The pump no longer reaches its usual ultimate pressure
- Unusual noise, higher running temperature, or hard starting
- Noticeable thinning or thickening of the drained oil compared with fresh oil
Any of these means it is time to drain, flush if necessary, and refill with fresh oil of the grade specified for your pump.
Conclusion
So, what is the viscosity of vacuum pump oil? In practice it ranges from ISO VG 22 to ISO VG 150 — about 22 to 150 cSt at 40 °C — with ISO VG 68 serving as the standard grade for most oil-sealed rotary vane pumps. The right choice depends on your pump's design, start-up temperature, and workload, and the pump manual should always have the final word.
InPowerVac (Zhejiang Yingpa Electromechanical Co., Ltd) manufactures a full range of oil-sealed rotary vane vacuum pumps and supplies dedicated vacuum pump oil, vanes, filters, and spare parts to keep them running at their rated performance. If you are unsure which viscosity grade fits your pump or application, contact our team for a tailored recommendation.
