ISO 32 vs ISO 46 Hydraulic Oil: What Is the Difference & Which Should You Use?
Updated: 24 September 2026
The main difference between ISO VG 32 and ISO VG 46 hydraulic oil is viscosity. ISO VG 32 has a nominal kinematic viscosity of approximately 32 mm²/s (cSt) at 40°C, while ISO VG 46 is approximately 46 mm²/s (cSt) at 40°C.
That does not mean ISO 32 is automatically the right oil for cold weather or light machinery, or that ISO 46 should automatically be used in warmer conditions, heavy machinery or higher-pressure systems.
The correct hydraulic oil should be selected according to the machine manufacturer’s requirements, required fluid specification and expected operating viscosity and temperature range.
ISO VG 32 is less viscous than ISO VG 46 at the same reference temperature. ISO VG 32 has a nominal viscosity of about 32 cSt at 40°C, compared with about 46 cSt for ISO VG 46. Neither grade is universally better. Use the viscosity grade and hydraulic-fluid specification required by the equipment manufacturer and application.
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ISO 32 vs ISO 46: Quick Comparison
| Characteristic | ISO VG 32 | ISO VG 46 |
|---|---|---|
| Nominal viscosity at 40°C | Approximately 32 cSt | Approximately 46 cSt |
| Relative viscosity | Lower | Higher |
| Is it a complete oil specification? | No | No |
| Automatically for cold conditions? | No | No |
| Automatically for heavy machinery? | No | No |
| Can it automatically replace the other grade? | No | No |
| Primary selection reference | OEM/application requirement | OEM/application requirement |
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?
ISO VG stands for ISO Viscosity Grade.
The system groups industrial lubricants according to their kinematic viscosity at a reference temperature of 40°C.
For the two grades being compared here:
- ISO VG 32 — nominal kinematic viscosity approximately 32 mm²/s (cSt) at 40°C
- ISO VG 46 — nominal kinematic viscosity approximately 46 mm²/s (cSt) at 40°C
This means ISO VG 46 is more viscous than ISO VG 32 when compared at the same reference temperature.
An ISO viscosity grade tells you about viscosity. It does not by itself tell you the complete additive package, base oil, anti-wear performance, oxidation resistance, viscosity index, compatibility or whether the fluid meets the specification required by your machine.
What Is ISO VG 32 Hydraulic Oil?
ISO VG 32 hydraulic oil belongs to a lower viscosity grade than ISO VG 46.
At the 40°C reference temperature, its nominal kinematic viscosity is approximately 32 cSt.
Whether that makes it suitable for a particular hydraulic system depends on the equipment requirements and the viscosity the oil will reach during actual operation.
It should therefore not be described simply as an oil for forklifts, agricultural machinery, indoor machinery or “light-duty” hydraulics. Equipment within any of those categories can have different fluid requirements.
What Is ISO VG 46 Hydraulic Oil?
ISO VG 46 hydraulic oil has a higher viscosity than ISO VG 32 at the same reference temperature.
Its nominal kinematic viscosity at 40°C is approximately 46 cSt.
ISO VG 46 is used in many hydraulic applications, but it should not automatically be described as the correct oil for excavators, piling rigs, presses or other heavy equipment simply because those machines operate under significant load.
The machine manufacturer determines the required viscosity and fluid specification according to the hydraulic system design and expected operating conditions.
Is ISO 46 Better for High-Pressure Hydraulic Systems?
Not automatically.
System pressure and oil viscosity are different engineering considerations.
A hydraulic system operating at high pressure does not automatically require ISO VG 46 instead of ISO VG 32.
Hydraulic components are designed to operate with an appropriate viscosity range, and the correct oil needs to satisfy the requirements of the pump, valves, actuators and wider hydraulic circuit.
Pressure alone should therefore not be used to choose between ISO VG 32 and ISO VG 46.
Is ISO 32 Better in Cold Weather?
ISO VG 32 is less viscous than ISO VG 46 at the same temperature, so viscosity grade can be relevant to cold-start behaviour.
However, the simple rule “ISO 32 for winter and ISO 46 for summer” is not reliable enough for hydraulic oil selection.
Cold-temperature performance also depends on characteristics beyond the ISO VG number, including the oil formulation and viscosity-temperature behaviour.
The machine manufacturer’s permitted viscosity range and expected start-up and operating temperatures should be considered.
How Temperature Changes Hydraulic Oil Viscosity
Hydraulic oil does not remain at one viscosity while a machine operates.
In general:
- As oil temperature falls, viscosity increases.
- As oil temperature rises, viscosity decreases.
This is one reason why hydraulic oil selection needs to consider the machine’s actual operating temperature range, not simply the ambient temperature outside.
A machine operating outdoors on a cool day may still develop substantial oil temperature during operation.
Likewise, cold-start requirements may be important even if the system later reaches a much higher operating temperature.
What Happens if Hydraulic Oil Is Too Viscous?
Oil that is too viscous for the operating conditions can contribute to problems such as:
- Increased resistance to flow
- Poor cold-start behaviour
- Sluggish hydraulic response
- Increased pressure losses
- Poor pump inlet conditions
- Increased risk of cavitation in unsuitable conditions
- Additional energy losses and heat generation
This is why choosing a “thicker” oil is not automatically better for hydraulic component protection.
What Happens if Hydraulic Oil Is Too Thin?
Oil with insufficient viscosity at operating temperature can also cause problems.
Possible effects include:
- Reduced lubricating film thickness
- Increased internal leakage
- Reduced volumetric efficiency
- Increased component wear
- Reduced protection of lubricated surfaces
The objective is therefore not to choose the thickest or thinnest oil possible. It is to keep the hydraulic fluid within the viscosity range required by the system during operation.
Does ISO 46 Stop Internal Hydraulic Leakage Better Than ISO 32?
A more viscous oil can affect internal leakage, but changing from ISO VG 32 to ISO VG 46 should not be used as a general repair for a worn hydraulic system.
Excessive internal leakage may indicate wear or damage within components such as:
- Hydraulic pumps
- Control valves
- Hydraulic cylinders
- Motors
If a machine has lost performance, the underlying cause should be diagnosed rather than simply changing to a more viscous oil in an attempt to compensate for wear.
See our Hydraulic System Troubleshooting & Diagnostics guide.
Can I Use ISO 32 Instead of ISO 46?
Do not substitute one viscosity grade for another without confirming that the alternative is permitted for the equipment and operating conditions.
Changing from ISO VG 46 to ISO VG 32 reduces viscosity at a given temperature. That can affect lubrication, internal leakage, efficiency and component behaviour.
Equally, changing from ISO VG 32 to ISO VG 46 increases viscosity and can affect cold-start behaviour, flow resistance and pump inlet conditions.
The correct answer therefore comes from the machine or component requirements rather than a universal substitution rule.
If a hydraulic system has become slow, noisy, hot or weak, changing from ISO 32 to ISO 46 — or vice versa — without diagnosing the problem can mask symptoms or create additional problems. Check the system and required oil specification first.
Can ISO 32 and ISO 46 Hydraulic Oil Be Mixed?
You should not assume that ISO VG 32 and ISO VG 46 hydraulic oils are safe to mix simply because both are described as hydraulic oil.
ISO VG describes viscosity, not the complete formulation.
Two oils can have different:
- Base oils
- Additive packages
- Anti-wear chemistry
- Detergent or dispersant characteristics
- Water-handling properties
- Material compatibility
- Performance specifications
Mixing also changes the resulting viscosity.
Before topping up with a different product, confirm compatibility and the equipment requirements.
See our guide: Can You Mix ISO 32 and ISO 46 Hydraulic Oil?
Should I Change to ISO 32 if My Hydraulic System Is Overheating?
Not without diagnosing why the system is running hot.
Hydraulic overheating can have many causes, including:
- Internal leakage
- Pressure-control problems
- Restricted flow
- Cooling problems
- Incorrect fluid level
- Incorrect viscosity
- Pump wear
- Valve faults
- Excessive pressure losses
- Operating conditions
Changing viscosity without identifying the cause can fail to solve the fault and may introduce another operating problem.
Read our Hydraulic Oil Temperature & Grades guide.
What About HVI Hydraulic Oil?
Some hydraulic fluids are formulated with a higher viscosity index to provide different viscosity-temperature characteristics across a wider temperature range.
These fluids can be useful in suitable applications where equipment experiences substantial temperature variation.
However, an HVI-type oil should not automatically replace another hydraulic fluid solely because the equipment operates outdoors.
The required specification and compatibility still need to be confirmed.
How Do I Choose Between ISO 32 and ISO 46?
A useful selection process is:
- Check the machine manual or manufacturer’s specification.
- Identify the required hydraulic-fluid type and performance specification.
- Confirm the permitted viscosity grade or operating viscosity range.
- Consider expected start-up and operating temperatures.
- Consider the requirements of the pump and other hydraulic components.
- Check compatibility before changing or topping up an existing fluid.
- Investigate abnormal heat, noise or performance rather than attempting to correct it by guessing a different viscosity.
If the manufacturer specifies ISO VG 32, ISO VG 46 or another approved viscosity grade for the operating conditions, follow that requirement unless an authorised engineering assessment establishes a reason to do otherwise.
ISO 32 vs ISO 46: Common Myths
| Common Claim | More Accurate Answer |
|---|---|
| ISO 32 is for winter | Not universally. Temperature matters, but OEM requirements and the oil’s wider characteristics also matter. |
| ISO 46 is for summer | Not universally. Selection should reflect the system and actual operating conditions. |
| Heavy machinery needs ISO 46 | Machine size alone does not determine the required viscosity grade. |
| High pressure means thicker oil | Pressure alone does not determine the correct ISO viscosity grade. |
| Thicker oil gives better protection | Excessive viscosity can itself create operating problems. |
| Any ISO 46 oil is equivalent | ISO VG 46 describes viscosity grade, not the complete fluid specification. |
Hydraulic Oil Supply, Filtration & Maintenance
Correct oil selection is only one part of hydraulic-fluid management.
Fluid condition can also be affected by:
- Solid-particle contamination
- Water ingress
- Excessive operating temperature
- Oxidation and degradation
- Incorrect topping-up procedures
- Mixing incompatible fluids
- Poor storage and transfer practices
Completely Hydraulic supports suitable hydraulic oil, filtration and system-maintenance requirements alongside hydraulic hose, cylinder, valve, pump and system engineering.
See our Hydraulic Oil Change & Filtration Service.
Related Hydraulic Oil Guides
- ISO 32, 46 & 68 Hydraulic Oil Explained
- Hydraulic Oil Viscosity Chart
- What Happens if You Use the Wrong Hydraulic Oil?
- Hydraulic Oil Guide: Types, Lifespan & Maintenance
- Signs Hydraulic Oil Needs Changing
Frequently Asked Questions
What is the difference between ISO 32 and ISO 46 hydraulic oil?
The primary difference is viscosity. ISO VG 32 has a nominal kinematic viscosity of approximately 32 cSt at 40°C, while ISO VG 46 is approximately 46 cSt at 40°C. ISO VG 46 is therefore more viscous at the same reference temperature.
Is ISO 32 thinner than ISO 46?
Yes. ISO VG 32 has a lower viscosity than ISO VG 46 when compared at the same temperature.
Which is better, ISO 32 or ISO 46?
Neither is universally better. The correct grade depends on the hydraulic system, manufacturer requirements and operating conditions.
Can I use ISO 32 instead of ISO 46?
Do not substitute ISO VG 32 for ISO VG 46 unless the equipment requirements permit it. The change in viscosity can affect lubrication, leakage, pump behaviour and system performance.
Can I use ISO 46 instead of ISO 32?
Not automatically. ISO VG 46 is more viscous and may alter cold-start behaviour, flow resistance and other system characteristics. Confirm the permitted viscosity before changing grades.
Can ISO 32 and ISO 46 hydraulic oils be mixed?
Do not assume they are compatible simply because they are hydraulic oils. ISO VG describes viscosity rather than the complete formulation, and mixing also changes the resulting viscosity. Check product compatibility and equipment requirements first.
Is ISO 46 better for heavy machinery?
Not necessarily. The size or weight of a machine does not by itself determine the correct hydraulic oil viscosity. Use the manufacturer’s specified fluid requirements.
Is ISO 32 better for cold weather?
ISO VG 32 is less viscous than ISO VG 46 at the same temperature, but cold-weather suitability depends on more than the ISO VG number. Manufacturer requirements and the expected temperature range should be considered.
Does high hydraulic pressure require ISO 46?
No. High system pressure does not automatically mean ISO VG 46 is required. Viscosity selection should be based on the hydraulic components, equipment specification and operating conditions.
What should I do if I don’t know which hydraulic oil is already in my machine?
Avoid guessing or topping up with an arbitrary hydraulic oil. Check equipment documentation, service records, labels and available fluid information. If identification remains uncertain, further investigation may be required before adding or replacing fluid.
Need Help With Hydraulic Oil?
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