Updated: 2 September 2026
Hydraulic Oil ISO Grades Explained: ISO 32 vs 46 vs 68
ISO VG 32, ISO VG 46 and ISO VG 68 are hydraulic oil viscosity grades. The number represents the oil’s nominal kinematic viscosity in mm²/s, also called centistokes (cSt), at 40°C.
That means ISO 32 is less viscous at 40°C than ISO 46, while ISO 68 is more viscous.
However, the highest or lowest ISO number is not automatically better. The correct hydraulic oil must match the machine manufacturer’s requirements, operating temperatures, pump and component design, viscosity requirements and fluid specification.
If you are unsure which hydraulic oil your machine requires, use our Hydraulic Oil Advisor as an identification aid and confirm the final specification against the machine or component manufacturer’s requirements.
Quick Answer:
What is the difference between ISO 32, ISO 46 and ISO 68 hydraulic oil? They have different viscosity grades. ISO VG 32 has a nominal viscosity of 32 cSt at 40°C, ISO VG 46 is 46 cSt and ISO VG 68 is 68 cSt. The correct grade depends on the equipment manufacturer’s specification and the viscosity required under the machine’s actual operating conditions.
Not Sure Which Hydraulic Oil You Need?
Some machines require specialist or OEM-specific hydraulic oil. Using the wrong oil can cause serious damage.
Check Your Oil NowWhat Does ISO VG Mean in Hydraulic Oil?
ISO VG means ISO Viscosity Grade.
The ISO viscosity classification groups industrial lubricants according to their kinematic viscosity at 40°C.
For the three common hydraulic oil grades discussed here:
- ISO VG 32 – nominal viscosity of 32 mm²/s (cSt) at 40°C
- ISO VG 46 – nominal viscosity of 46 mm²/s (cSt) at 40°C
- ISO VG 68 – nominal viscosity of 68 mm²/s (cSt) at 40°C
One centistoke (cSt) is equivalent to one mm²/s.
Engineering Point
The ISO VG number is a viscosity classification — not a complete hydraulic oil specification. Two ISO VG 46 oils can have the same nominal viscosity grade while differing in base oil, additive package, viscosity index, anti-wear performance, approvals and compatibility.
ISO 32 vs ISO 46 vs ISO 68 Hydraulic Oil
| ISO Grade | Nominal Viscosity at 40°C | Relative Viscosity | Selection Principle |
|---|---|---|---|
| ISO VG 32 | 32 cSt | Lower than ISO 46 and 68 | Use where specified for the equipment and operating conditions |
| ISO VG 46 | 46 cSt | Between ISO 32 and 68 | Use where specified for the equipment and operating conditions |
| ISO VG 68 | 68 cSt | Higher than ISO 32 and 46 | Use where specified for the equipment and operating conditions |
For a wider viscosity comparison, see our hydraulic oil viscosity chart.
Why Does Hydraulic Oil Viscosity Matter?
Hydraulic oil has to perform several jobs at once. It transfers power while also lubricating components, helping control wear and carrying heat through the system.
Viscosity affects how easily the fluid flows and how effectively it separates and lubricates moving surfaces.
If Hydraulic Oil Is Too Viscous
Oil that is too viscous for the equipment or operating conditions may contribute to:
- Higher resistance to flow
- Poor cold-start behaviour
- Sluggish hydraulic response
- Pump inlet difficulties
- Increased risk of cavitation under unsuitable inlet conditions
- Higher energy losses
If Hydraulic Oil Is Not Viscous Enough
Oil that is too low in viscosity under operating conditions may contribute to:
- Reduced lubricating film thickness
- Higher internal leakage
- Reduced volumetric efficiency
- Increased component wear
- Reduced system performance
The Machine Operates at Operating Temperature — Not 40°C Forever
ISO viscosity grades are classified at 40°C, but hydraulic oil viscosity continually changes with temperature. The important engineering question is whether the oil provides suitable viscosity throughout the machine’s actual start-up and operating temperature range.
Is ISO 32 Hydraulic Oil for Cold Weather?
ISO VG 32 is less viscous than ISO VG 46 and ISO VG 68 at the same reference temperature, so it may be specified for some equipment operating under cooler conditions.
That does not mean every machine should switch to ISO 32 during winter.
The correct cold-temperature performance also depends on:
- OEM requirements
- Oil viscosity index
- Minimum start-up temperature
- Pump inlet requirements
- Actual operating temperature
- Fluid formulation
Read our guide to hydraulic oil for cold and hot weather for more detail.
Is ISO 46 the Most Common Hydraulic Oil?
ISO VG 46 is widely used in many industrial and mobile hydraulic applications, but that does not make it the correct default for every machine.
A system designed for ISO VG 32 or ISO VG 68 should not automatically be filled with ISO VG 46 simply because it is commonly available.
Do Not Guess the Grade
If the required oil is unknown, check the machine manual, reservoir label, component specification or manufacturer information. Using a commonly available oil as a substitute without checking compatibility can create new hydraulic problems.
Is ISO 68 Hydraulic Oil for Hot or Heavy-Duty Systems?
ISO VG 68 is more viscous than ISO VG 46 and ISO VG 32 at the same reference temperature.
It may be specified in equipment where a higher viscosity grade is required, including some systems operating under particular temperature or load conditions.
However, heavy load does not automatically mean ISO VG 68 should be used. The machine and component manufacturer’s viscosity requirements remain the deciding factor.
How Temperature Changes Hydraulic Oil Viscosity
Hydraulic oil generally becomes:
- More viscous as temperature falls
- Less viscous as temperature rises
This is why a hydraulic oil that performs correctly at normal operating temperature may behave very differently during a cold start.
The oil’s viscosity index (VI) is another important property. In general terms, viscosity index indicates how strongly an oil’s viscosity changes as temperature changes.
Therefore, oil selection should consider more than simply the ISO VG number.
How Do You Choose the Correct Hydraulic Oil Grade?
01 — CHECK THE OEM SPECIFICATION
Start with the machine manual, manufacturer data or component requirements.
02 — IDENTIFY THE ISO VG GRADE
Confirm whether the equipment requires ISO VG 32, 46, 68 or another viscosity grade.
03 — CHECK THE REQUIRED FLUID TYPE
Confirm additive, anti-wear, fire-resistant, biodegradable or other special fluid requirements where applicable.
04 — CONSIDER TEMPERATURE
Make sure the fluid can provide suitable viscosity during cold start and normal operating temperatures.
05 — CONFIRM COMPATIBILITY
Do not substitute or mix hydraulic oils until compatibility and specification have been checked.
Does the Hydraulic Pump Affect Which Oil You Need?
Yes. Pumps and other hydraulic components are designed to operate within specified viscosity ranges.
Oil that becomes excessively viscous at the pump inlet can contribute to poor filling and cavitation risk, while oil that becomes too low in viscosity can increase internal leakage and reduce lubrication.
This is one reason an oil grade should be selected using the equipment or component manufacturer’s requirements rather than a generic temperature chart alone.
Can You Mix ISO 32 and ISO 46 Hydraulic Oil?
Do not assume that two hydraulic oils are safe to mix just because their ISO viscosity grades are similar.
Compatibility can depend on:
- Base oil type
- Additive chemistry
- OEM approvals
- Fluid condition
- Required final viscosity
- Special system requirements
Mixing grades also changes the viscosity of the resulting fluid.
Read our dedicated guide: Can You Mix ISO 32 and ISO 46 Hydraulic Oil?
What Happens If You Use the Wrong Hydraulic Oil Grade?
Using unsuitable hydraulic oil can affect several areas of system performance.
Depending on whether the fluid is too viscous, insufficiently viscous, chemically unsuitable or otherwise incorrectly specified, possible problems include:
- Slow operation
- Poor cold starting
- Pump cavitation
- Excessive internal leakage
- Reduced lubrication
- Increased wear
- Higher operating temperature
- Seal compatibility problems
- Reduced hydraulic efficiency
See what happens if you use the wrong hydraulic oil for the full guide.
Correct Oil = Correct Specification
Choosing hydraulic oil is not simply a decision between 32, 46 and 68. The correct fluid must provide the viscosity, chemistry and approvals required by the machine throughout its operating conditions.
Can You Change From ISO 46 to ISO 32 or ISO 68?
Only where the equipment requirements support the change.
Do not change viscosity grade simply because:
- The weather has changed
- Another grade is cheaper
- Another oil is already in stock
- The machine seems to be running hot
- A different grade is used in another machine
A heat problem, for example, may be caused by internal leakage, restrictions, cooling faults or hydraulic inefficiency rather than the viscosity grade itself.
If system behaviour has changed, diagnosis should identify the cause before changing fluid specification.
How Do I Know What Hydraulic Oil Is Already in My Machine?
Useful sources include:
- Machine manufacturer’s manual
- Reservoir or service labels
- Maintenance records
- Previous oil purchase records
- Component manufacturer’s information
- Fluid sampling and analysis where necessary
If the existing oil cannot be positively identified, avoid simply adding a different product without checking compatibility.
Try the Completely Hydraulic Oil Advisor
Our Hydraulic Oil Advisor provides a useful starting point when identifying oil requirements for hydraulic equipment.
It does not replace the machine manufacturer’s specification, but it can help you work through the information needed to identify an appropriate fluid.
Not Sure Which Hydraulic Oil You Need?
Check the machine information and use our Hydraulic Oil Advisor to help identify the required grade and specification.
Related Hydraulic Oil Guides
- Hydraulic Oil Viscosity Chart
- Hydraulic Oil for Cold vs Hot Weather
- Can You Mix ISO 32 and ISO 46 Hydraulic Oil?
- What Happens If You Use the Wrong Hydraulic Oil?
- Hydraulic Oil Guide: Types, Lifespan & Maintenance
- Signs Hydraulic Oil Needs Changing
- Hydraulic Oil Change & Filtration
- Emergency Hydraulic Oil Supply
Need Help Choosing Hydraulic Oil?
If you are unsure whether your machine requires ISO 32, ISO 46, ISO 68 or a specialist hydraulic fluid, check the equipment specification or speak to Completely Hydraulic.
Hydraulic Oil ISO Grade FAQs
What does ISO 32 hydraulic oil mean?
ISO VG 32 is a viscosity grade with a nominal kinematic viscosity of 32 mm²/s, or cSt, at 40°C. Whether it is suitable depends on the hydraulic equipment and operating conditions.
What does ISO 46 hydraulic oil mean?
ISO VG 46 is a viscosity grade with a nominal kinematic viscosity of 46 cSt at 40°C. It is widely used in hydraulic equipment, but should only be used where it meets the machine or component specification.
What does ISO 68 hydraulic oil mean?
ISO VG 68 is a viscosity grade with a nominal kinematic viscosity of 68 cSt at 40°C. It is more viscous than ISO VG 46 or ISO VG 32 at the same reference temperature.
Is ISO 32 thinner than ISO 46?
Yes. At the same reference temperature, ISO VG 32 has a lower viscosity than ISO VG 46.
Is ISO 68 thicker than ISO 46?
Yes. At the same reference temperature, ISO VG 68 is more viscous than ISO VG 46.
Is ISO 46 the best hydraulic oil for UK weather?
There is no universal hydraulic oil grade for UK weather. ISO VG 46 is widely used, but the correct oil depends on the machine manufacturer’s specification, operating temperature, viscosity requirements and fluid type.
Should I use ISO 32 hydraulic oil in winter?
Not automatically. ISO VG 32 is less viscous than ISO VG 46 and ISO VG 68, but any seasonal grade change should follow the equipment manufacturer’s recommendations and operating viscosity requirements.
Can I use ISO 68 instead of ISO 46?
Only if the machine or component specification permits it. ISO VG 68 is more viscous and can change cold-start behaviour, flow characteristics and pump inlet conditions.
Can you mix ISO 32 and ISO 46 hydraulic oil?
Do not assume they are compatible simply because both are hydraulic oils. Check the products’ chemistry, approvals and machine requirements before mixing because the resulting blend will also have a different viscosity.
How do I find the correct hydraulic oil for my machine?
Start with the machine manual, manufacturer specification, reservoir labels or component documentation. Confirm both the required viscosity grade and the hydraulic fluid specification before filling or topping up the system.