Hydraulic Oil Viscosity Chart: ISO 32 vs 46 vs 68
Compare ISO VG 32, ISO VG 46 and ISO VG 68 hydraulic oil, understand what the viscosity numbers actually mean, and see how temperature affects hydraulic oil selection.
Last Updated: 27 August 2026
Hydraulic oil viscosity affects how easily fluid flows, how effectively moving components are lubricated and how a hydraulic system behaves as temperature changes.
The familiar grades ISO VG 32, ISO VG 46 and ISO VG 68 refer to different viscosity classes measured around a nominal viscosity at 40°C.
ISO 32 is less viscous than ISO 46, while ISO 68 is more viscous than both.
However, that does not mean that ISO 32 is automatically the correct winter oil or ISO 68 automatically the correct hot-weather oil for every machine.
The correct hydraulic oil should be selected using the machine manufacturer’s specification, required viscosity range, operating temperature and oil specification.
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 NowQuick Answer: What Is the Difference Between ISO 32, 46 and 68 Hydraulic Oil?
The main difference is viscosity.
ISO VG 32 has a nominal kinematic viscosity of approximately 32 mm²/s (cSt) at 40°C, ISO VG 46 approximately 46 cSt, and ISO VG 68 approximately 68 cSt.
That means ISO 32 flows more easily at the same temperature, while ISO 68 is more resistant to flow.
The correct grade is the one that keeps the oil within the hydraulic equipment manufacturer’s required viscosity range during actual operation.
Not Sure Which Hydraulic Oil Grade You Need?
Use our Hydraulic Oil Advisor as a starting point, then confirm the required specification against the machine manufacturer’s information before changing or topping up the system.
What Does Hydraulic Oil Viscosity Mean?
Viscosity describes a fluid’s resistance to flow.
In practical terms:
- A lower-viscosity oil flows more easily
- A higher-viscosity oil is more resistant to flow
Hydraulic oil needs sufficient viscosity to help maintain lubrication and sealing between moving components, but it must also flow freely enough through pumps, valves, hoses and passages.
Why Viscosity Matters
- Oil that is too thin can increase internal leakage and reduce lubricating film thickness
- Oil that is too thick can increase resistance to flow and make cold starting more difficult
- Correct operating viscosity helps the pump, valves and actuators operate within their intended conditions
Hydraulic Oil Viscosity Chart: ISO 32 vs ISO 46 vs ISO 68
| ISO Grade | Nominal Viscosity at 40°C | Relative Thickness | General Selection Tendency |
|---|---|---|---|
| ISO VG 32 | Approx. 32 cSt | Lower | Often considered where lower-temperature operation or a lower viscosity is specified |
| ISO VG 46 | Approx. 46 cSt | Medium | Widely encountered in industrial and mobile hydraulic applications |
| ISO VG 68 | Approx. 68 cSt | Higher | May be specified where higher operating viscosity is required |
The table provides a comparison of viscosity grades, not a universal machine-selection chart. Always confirm the required grade and specification for the particular hydraulic equipment.
What Does ISO VG Mean on Hydraulic Oil?
ISO VG means ISO Viscosity Grade.
The number identifies the oil’s nominal kinematic viscosity in millimetres squared per second — also commonly written as centistokes or cSt — at 40°C.
Simple Example
ISO VG 46 does not mean the oil should operate at 46°C. It means the oil belongs to the ISO viscosity grade centred around approximately 46 cSt at the standard 40°C measurement temperature.
How Does Temperature Affect Hydraulic Oil Viscosity?
Hydraulic oil does not stay at the same viscosity as temperature changes.
As oil becomes hotter, its viscosity normally falls.
As oil becomes colder, its viscosity normally rises.
COLDER OIL
Viscosity increases and the oil becomes more resistant to flow.
OPERATING TEMPERATURE
The oil should remain within the viscosity range required by the pump and other hydraulic components.
HOTTER OIL
Viscosity decreases, which can reduce lubricating-film thickness and increase internal leakage if the oil becomes too thin for the system.
This is why the grade printed on the drum cannot be considered in isolation from operating temperature.
For more detail, see our
hydraulic oil temperature guide.
What Is the Viscosity Index of Hydraulic Oil?
The viscosity index (VI) describes how much an oil’s viscosity changes as temperature changes.
Two hydraulic oils can both be ISO VG 46 at 40°C but behave differently across a wide operating-temperature range if their viscosity indices are different.
Why This Matters
ISO VG tells you the viscosity class at the reference temperature. It does not, by itself, tell you exactly how the oil will behave at every temperature the machine experiences.
When Is ISO 32 Hydraulic Oil Used?
ISO VG 32 has a lower viscosity than ISO VG 46 and ISO VG 68 at the same temperature.
It may be specified in hydraulic equipment where:
- Lower-temperature operation is expected
- The component manufacturer requires a lower viscosity grade
- Cold-start behaviour is an important consideration
- The system has been designed around ISO VG 32
However, these are general tendencies rather than a reason to substitute ISO 32 into a machine specified for another fluid.
When Is ISO 46 Hydraulic Oil Used?
ISO VG 46 is widely encountered in industrial and mobile hydraulic equipment.
Its nominal viscosity sits between ISO 32 and ISO 68, making it a common specification across many hydraulic systems operating in moderate conditions.
That does not make ISO 46 universally correct.
A machine designed for ISO 32, ISO 68, a high-viscosity-index fluid, a fire-resistant fluid, biodegradable fluid or another OEM-specific specification should continue to follow the appropriate requirement.
When Is ISO 68 Hydraulic Oil Used?
ISO VG 68 has a higher viscosity than ISO VG 46 or ISO VG 32 at the same temperature.
It may be specified in systems where a higher operating viscosity is required due to equipment design or operating conditions.
But simply changing to a thicker oil because a machine is running hot can be the wrong approach.
A Hot Hydraulic System Needs Diagnosis
If a hydraulic system is overheating, investigate the cause. Changing from ISO 46 to ISO 68 without confirming the machine requirements can mask the problem or create additional flow and cold-start issues.
ISO 32 vs ISO 46 vs ISO 68: Which Is Thicker?
At the same temperature:
ISO 32 → ISO 46 → ISO 68
moves from lower viscosity to higher viscosity.
ISO 32
Lowest viscosity of the three.
ISO 46
Higher viscosity than ISO 32 but lower than ISO 68.
ISO 68
Highest viscosity of the three.
For the full comparison, read:
ISO 32 vs 46 vs 68 Hydraulic Oil Explained.
What Hydraulic Oil Viscosity Do Excavators Use?
There is no single hydraulic oil grade that should be used in every excavator.
ISO VG 46 is commonly encountered in plant and hydraulic equipment, but excavator manufacturers may specify different viscosity grades or dedicated hydraulic fluids according to machine design and expected ambient temperature.
Before topping up or replacing oil, check:
- The operator or service manual
- Manufacturer fluid specification
- Required viscosity grade
- Ambient and operating-temperature range
- The fluid already present in the system
See:
What Happens If Hydraulic Oil Is Too Thick?
If hydraulic oil viscosity is too high for the system and temperature, possible effects can include:
- Sluggish hydraulic response
- Increased resistance to flow
- Difficult cold starting
- Poor pump inlet conditions
- Increased energy losses
What Happens If Hydraulic Oil Is Too Thin?
If viscosity becomes too low for the hydraulic components, possible effects can include:
- Increased internal leakage
- Reduced lubricating-film thickness
- Reduced volumetric efficiency
- Accelerated wear under unsuitable operating conditions
Important Diagnostic Point
Slow operation, pump noise or overheating can have several causes. Do not assume the oil viscosity is responsible until the wider hydraulic system has been considered.
Can the Wrong Hydraulic Oil Look Like a Mechanical Fault?
Yes.
Incorrect viscosity can influence hydraulic response, pump behaviour and system efficiency.
However, similar symptoms can also result from:
- Pump wear
- Valve faults
- Restricted filters
- Internal leakage
- Contamination
- Cooling problems
- Incorrect pressure settings
For oil-condition warning signs, read:
Signs Your Hydraulic Oil Needs Changing.
Can You Mix ISO 32, ISO 46 and ISO 68 Hydraulic Oil?
Do not assume that hydraulic oils are compatible simply because they use the same ISO VG classification system.
Different products can contain different:
- Base oils
- Additive packages
- Anti-wear chemistry
- OEM approvals
- Viscosity modifiers
Mixing different viscosity grades will also change the resulting viscosity.
Before Mixing Hydraulic Oils
Identify the existing fluid and confirm product compatibility and machine requirements. If the oil already in the system is unknown, further investigation may be more appropriate than simply topping it up with another grade.
How Should You Choose Hydraulic Oil Viscosity?
01 — CHECK THE OEM SPECIFICATION
Start with the machine or component manufacturer’s required oil specification.
02 — IDENTIFY THE REQUIRED VISCOSITY
Confirm the required ISO VG grade or permitted viscosity range.
03 — CONSIDER TEMPERATURE
Think about both cold start-up and normal operating temperature.
04 — CHECK THE OIL SPECIFICATION
Viscosity grade alone does not define additive type, base oil or OEM approval.
05 — CONFIRM COMPATIBILITY
Before topping up or changing oil, establish what fluid is already in the hydraulic system.
Hydraulic Oil Viscosity: Quick Summary
ISO VG 32
Nominal viscosity of approximately 32 cSt at 40°C.
ISO VG 46
Nominal viscosity of approximately 46 cSt at 40°C.
ISO VG 68
Nominal viscosity of approximately 68 cSt at 40°C.
TEMPERATURE
Oil becomes more viscous as it cools and less viscous as it heats.
VISCOSITY INDEX
Different oils within the same ISO grade can behave differently as temperature changes.
BEST RULE
Use the machine manufacturer’s viscosity and fluid specification rather than choosing an oil by ISO number alone.
Need Hydraulic Oil or Help Identifying the Correct Grade?
Completely Hydraulic supports customers requiring hydraulic oil and related hydraulic products through its engineering and trade-counter network.
If you need oil for a machine, try to provide:
- Machine make and model
- Manufacturer fluid specification
- Current hydraulic oil grade if known
- Approximate quantity required
- Operating environment
For urgent requirements, see our
hydraulic oil supply information.
Related Hydraulic Oil Guides
Need Help Choosing Hydraulic Oil?
Use our Hydraulic Oil Advisor or speak to Completely Hydraulic if you need help identifying an appropriate oil grade or sourcing hydraulic oil for your machinery.
Hydraulic Oil Viscosity Chart FAQs
What does a hydraulic oil viscosity chart show?
A hydraulic oil viscosity chart compares oil viscosity grades and helps explain how viscosity changes with temperature. It can be used alongside manufacturer specifications when selecting hydraulic oil.
What does ISO VG 46 mean?
ISO VG 46 means the oil belongs to the ISO viscosity grade centred around approximately 46 mm²/s, or 46 cSt, at 40°C.
Is ISO 46 thicker than ISO 32 hydraulic oil?
Yes. At the same temperature, ISO VG 46 has a higher viscosity than ISO VG 32.
Is ISO 68 thicker than ISO 46?
Yes. ISO VG 68 has a higher viscosity than ISO VG 46 at the same temperature.
Is ISO 32 better for cold weather?
Its lower viscosity can make ISO VG 32 more suitable for some lower-temperature applications, but the correct grade depends on the machine manufacturer’s specification, oil viscosity index and actual operating temperature.
Is ISO 68 better for hot weather?
ISO VG 68 provides higher viscosity at the same temperature and may be specified in some hotter operating conditions. It should not automatically be substituted for ISO 46 or ISO 32 without checking equipment requirements.
Can I use ISO 46 instead of ISO 32?
Not automatically. Changing from ISO 32 to ISO 46 increases viscosity and may affect cold starting, flow and component performance. Check the manufacturer’s permitted grades before changing oil.
Can I use ISO 68 instead of ISO 46?
Only where the equipment manufacturer or relevant technical specification permits it. ISO 68 is more viscous and can behave differently during cold start-up and normal operation.
What happens to hydraulic oil viscosity when it gets hot?
Hydraulic oil normally becomes less viscous as temperature rises. If viscosity becomes too low, lubrication and internal sealing within pumps and other components can be affected.
What happens to hydraulic oil when it gets cold?
Hydraulic oil becomes more viscous as it cools. If it becomes too thick for the equipment, start-up and fluid flow can become more difficult.
What is the best hydraulic oil grade?
There is no universal best grade. The correct oil is the one that meets the machine manufacturer’s viscosity, performance and fluid specification across the expected operating conditions.