Introduction
Hydraulic systems depend on fluid to transmit power, lubricate moving components and help manage heat. ISO VG 32, 46 and 68 are common hydraulic oil viscosity grades. Their numbers describe viscosity at a reference temperature; they do not indicate a ranking of oil quality.
What does ISO VG mean?
ISO VG stands for International Organization for Standardization viscosity grade. Under ISO 3448, the grade number represents a nominal kinematic viscosity in mm²/s (centistokes, cSt) at 40°C. Each grade has a permitted viscosity range of ±10% around its nominal midpoint.
ISO VG 32 vs 46 vs 68: comparison
At 40°C, ISO VG 32 has the lowest viscosity of the three grades and ISO VG 68 the highest. The table shows the ISO 3448 classification ranges, not guaranteed measurements for a particular product.
| ISO grade | Nominal viscosity at 40°C | ISO 3448 range at 40°C |
| VG 32 | 32 mm²/s (cSt) | 28.8–35.2 mm²/s |
| VG 46 | 46 mm²/s (cSt) | 41.4–50.6 mm²/s |
| VG 68 | 68 mm²/s (cSt) | 61.2–74.8 mm²/s |
Why hydraulic oil viscosity matters
Hydraulic oil must remain within the equipment manufacturer’s acceptable viscosity range across startup and normal operation. Oil that is too viscous for the conditions may hinder flow and increase resistance at startup. Oil that becomes too low in viscosity may increase internal leakage and reduce lubrication-film thickness. Actual system effects depend on the pump, components and operating conditions.
How temperature affects hydraulic oil
Most hydraulic oils become less viscous as temperature increases and more viscous as temperature falls. An ISO VG number is measured at 40°C, not at the equipment’s actual operating temperature. Viscosity index, base-oil formulation and the system’s operating range influence how the fluid behaves across temperatures.
Where are ISO VG 32, 46 and 68 used?
These grades are found across industrial and mobile hydraulic equipment, but there is no universal rule assigning a particular grade to a type of machine. A pump or system may require a specific grade or allow alternatives depending on temperature and operating conditions. Refer to the equipment manual for the applicable viscosity range and fluid specification.
Viscosity grade is not the same as fluid performance
ISO VG 46 does not, by itself, mean an oil is anti-wear, high-viscosity-index, fire-resistant or suitable for every hydraulic system. Fluid classifications and manufacturer specifications address performance characteristics beyond viscosity. Mineral, synthetic, biodegradable and fire-resistant hydraulic fluids may also have different compatibility requirements.
Can you replace ISO VG 46 with ISO VG 68?
Do not substitute grades solely because the environment is hot or the machine is heavily loaded. A different grade can affect cold-start behavior, pump inlet conditions and performance at operating temperature. Change grades only when the equipment manufacturer permits the alternative and the fluid meets the required specification.
How to choose the right hydraulic oil
Identify the manufacturer’s specified viscosity grade and fluid performance requirements. Check the operating temperature range, component and pump requirements, and the fluid’s current technical data sheet. Consider contamination control and maintenance practices separately: correct viscosity alone does not ensure reliable hydraulic-system operation.
Conclusion
ISO VG 32, 46 and 68 describe distinct hydraulic-oil viscosity classes at 40°C. The correct choice depends on the system’s specified fluid type and acceptable viscosity across its operating conditions, not on choosing the highest or lowest number.
Frequently Asked Questions
What is the difference between ISO VG 32, 46 and 68?
They are different nominal viscosity classes measured at 40°C: 32, 46 and 68 mm²/s respectively.
Is ISO VG 68 always better for hot weather?
No. Operating temperature matters, but the manufacturer’s allowable viscosity range and specified fluid type determine suitability.
Can I mix ISO VG 32 and 46 hydraulic oils?
Do not assume compatibility. Different grades and additive systems may not be suitable for mixing; consult the equipment and fluid manufacturers.
Does ISO VG 46 mean the oil is anti-wear?
No. ISO VG is a viscosity classification, not an anti-wear performance specification.
Is hydraulic oil the same as gear oil?
No. Lubricants are formulated for different applications and are not interchangeable unless the manufacturer expressly specifies a suitable multipurpose fluid.
