General educational information, not a product approval. Always confirm the current TDS, the equipment manual and the OEM requirement before ordering.
ISO VG 46 and 68 describe the oil's nominal viscosity at 40°C. VG 68 is thicker than VG 46. Both can be correct in different systems; the equipment manufacturer's requirement is the starting point.
Hydraulic oil must protect the pump, provide film strength and flow through the system at start-up and operating temperature. Using the wrong viscosity can increase wear or reduce efficiency.
At a glance
| Grade | Nominal viscosity at 40°C | Typical use | Watch-outs |
|---|---|---|---|
| ISO VG 46 | ≈46 mm²/s | Systems specified for 46; cooler start-up conditions | Confirm pump and start-up temperature |
| ISO VG 68 | ≈68 mm²/s | Systems specified for 68; warmer conditions | Higher viscosity can increase cold-start load |
Viscosity is not quality
A higher ISO VG number does not mean a better oil. It means a thicker oil at 40°C. The correct grade is the one that gives the required viscosity at the system's operating and start-up temperatures.
Temperature and pump
A hot system may need a higher viscosity grade to maintain film thickness, while a cold climate may require a lower grade for start-up. The pump manufacturer's minimum and maximum viscosity limits take priority.
What to check before ordering
Confirm the equipment model, pump type, oil grade in the manual, operating temperature range and start-up temperature. Ask for the TDS, including viscosity index, pour point and any OEM approval. Do not mix 46 and 68 to “adjust” viscosity.
Bottom line
Use the grade specified for the pump and operating conditions. Choose 46 or 68 based on temperature and the manual, not on a preference for thicker oil.