Hydraulic Oil in the UK: Grades, Uses & How to Choose
Understand what hydraulic oil does, how ISO viscosity grades such as 32, 46 and 68 differ, and what to check before putting oil into a hydraulic system.
Last Updated: 25 August 2026
Hydraulic oil is the fluid that transfers power through many hydraulic systems while also helping lubricate, cool and protect internal components.
Across the UK, hydraulic oils are used in industrial machinery, construction plant, agricultural equipment, material-handling systems, mobile hydraulics and specialist equipment.
Choosing the correct oil is important because viscosity, additive package, temperature range, manufacturer specification and fluid compatibility can all affect system performance and component life.
Common ISO viscosity grades include ISO VG 32, ISO VG 46 and ISO VG 68, but the correct grade depends on the equipment and operating conditions rather than simply choosing the most common product.
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: Which Hydraulic Oil Should You Use?
Use the hydraulic oil grade and specification recommended by the machine or hydraulic-component manufacturer wherever that information is available.
ISO VG 32, 46 and 68 describe viscosity grades rather than overall oil quality. A machine designed for ISO 46 should not automatically be changed to ISO 32 or ISO 68 without considering operating temperature, manufacturer guidance and the existing fluid.
Do not choose hydraulic oil by viscosity number alone. Check the required specification, fluid type and compatibility first.
Not Sure Which Hydraulic Oil You Need?
Different machines can require different viscosity grades, specifications and hydraulic-fluid types.
Use our oil selector as a starting point, then check the machine manufacturer’s specification before changing or topping up the system.
What Does Hydraulic Oil Do?
The primary job of hydraulic oil is to transmit energy through the hydraulic circuit.
The pump moves the fluid, valves control its direction and flow, and actuators such as cylinders or hydraulic motors convert that hydraulic energy into mechanical movement.
Depending on the system and fluid formulation, hydraulic oil also performs several other important functions:
- Lubrication – reducing friction and wear between moving components
- Cooling – carrying heat away from components and through the system
- Sealing assistance – helping maintain effective clearances within pumps, valves and actuators
- Contamination transport – carrying particles towards filtration
- Corrosion protection – depending on the additives used in the fluid
Engineering Point
Hydraulic oil is not simply something that fills the reservoir. Its viscosity and condition directly influence lubrication, leakage, heat generation and how efficiently hydraulic components operate.
Where Is Hydraulic Oil Used?
Hydraulic oil is used in equipment where pressurised fluid transfers energy to produce force or motion.
Applications can include:
- Construction machinery
- Excavators and loaders
- Industrial presses
- Manufacturing equipment
- Material-handling machinery
- Agricultural machinery
- Waste and recycling equipment
- Vehicle-mounted hydraulic systems
- Lifting equipment
- Special-purpose industrial systems
The actual fluid specification can vary significantly between these applications.
What Do ISO 32, ISO 46 and ISO 68 Mean?
ISO VG numbers describe the oil’s viscosity grade.
In simple terms, a lower ISO VG number represents a lower-viscosity oil at the standard reference temperature, while a higher number represents a more viscous oil.
| Grade | Relative Viscosity | Important Point |
|---|---|---|
| ISO VG 32 | Lower than ISO 46 and ISO 68 | May suit some lower-temperature or manufacturer-specified applications |
| ISO VG 46 | Mid-range of these three common grades | Widely encountered in industrial hydraulic equipment |
| ISO VG 68 | Higher than ISO 32 and ISO 46 | May be specified where higher viscosity is required |
These are viscosity classifications, not instructions to substitute one oil for another.
For a deeper explanation, see our hydraulic oil viscosity chart.
How Do You Choose the Correct Hydraulic Oil?
The best starting point is the machine manufacturer’s specification.
01 — CHECK THE MANUAL
Identify the manufacturer’s required hydraulic-fluid specification and viscosity grade.
02 — IDENTIFY THE EXISTING OIL
Find out what fluid is already in the system before topping up or changing grades.
03 — CONSIDER OPERATING CONDITIONS
Temperature, duty cycle, load and environment can influence viscosity requirements.
04 — CHECK COMPATIBILITY
Do not mix unknown or incompatible fluids simply because their viscosity grades appear similar.
What Should You Look for in Hydraulic Oil?
Viscosity is important, but it is only one part of hydraulic-fluid selection.
Depending on the oil type and specification, the formulation may include additives designed to provide:
- Anti-wear protection
- Oxidation resistance
- Rust and corrosion protection
- Foam control
- Air-release performance
- Water-separation characteristics
The required performance characteristics depend on the equipment and the hydraulic-system manufacturer’s specification.
Correct Specification Beats Guesswork
Two hydraulic oils can carry the same ISO viscosity grade but still have different additive packages or approvals. Matching the required specification matters as well as matching the viscosity.
Should You Use ISO 32 or ISO 46 Hydraulic Oil?
Neither grade is universally better.
The correct choice depends on the machine specification and operating conditions.
ISO 32 is less viscous than ISO 46, which may be useful in some cooler operating conditions or equipment specifically designed for that grade.
ISO 46 provides higher viscosity and is commonly specified in a wide range of industrial hydraulic systems.
Do Not Change Grades Without Checking
Changing from ISO 46 to ISO 32, or from ISO 32 to ISO 46, changes the fluid viscosity. Always check the manufacturer’s specification and consider the existing oil and operating temperature before changing grade.
See our detailed guide: ISO 32 vs ISO 46 hydraulic oil.
Can You Mix ISO 32 and ISO 46 Hydraulic Oil?
Mixing hydraulic oils should not be treated as automatically safe simply because both fluids are hydraulic oils.
Different products can contain different base oils, additives and performance specifications.
Even where two mineral hydraulic oils appear compatible, mixing ISO 32 and ISO 46 will also change the resulting viscosity.
If the existing fluid is unknown or a significant oil change is being considered, identify the fluid and check compatibility first.
Read the full guide: Can You Mix ISO 32 and ISO 46 Hydraulic Oil?
What Happens If You Use the Wrong Hydraulic Oil?
The effect depends on how different the incorrect fluid is from the required specification.
Possible problems can include:
- Poor cold-start performance
- Excessive internal leakage if viscosity is too low
- Higher flow resistance if viscosity is too high
- Increased heat generation
- Reduced lubrication
- Accelerated component wear
- Seal-compatibility problems with unsuitable fluid types
- Reduced hydraulic-system efficiency
Already Added the Wrong Oil?
Do not assume the machine should simply continue operating. Identify what was added, how much entered the system and what fluid should be present before deciding whether further action is required.
Why Does Hydraulic Oil Cleanliness Matter?
Even the correct hydraulic oil can cause problems if it becomes heavily contaminated.
Hydraulic components often contain small internal clearances, so particles, water and degraded oil can contribute to wear and unreliable operation.
Contamination can affect:
- Pumps
- Control valves
- Hydraulic cylinders
- Hydraulic motors
- Seals
- Bearings and internal surfaces
Filtration, correct storage, clean transfer equipment and sensible maintenance practices all help protect hydraulic fluid condition.
What Sizes Is Hydraulic Oil Supplied In?
Hydraulic oil can be supplied in different package sizes depending on the brand, product and supplier.
Common commercial formats can include:
- Smaller containers for topping up and maintenance
- 20 or 25 litre containers
- Drums, commonly around 200 litres
- Intermediate bulk containers for larger users
- Bulk supply for high-volume applications
Not every oil is available in every pack size, so availability should be checked for the specific product required.
Hydraulic Oil Supply in the UK
For industrial users, choosing the correct oil is only part of the requirement. Reliable supply and access to technical support can also matter when machinery needs to remain operational.
Completely Hydraulic supports customers through our hydraulic engineering and trade-counter network across London, Essex, Kent and Heathrow, alongside mobile hydraulic coverage in additional areas.
Our trade counters can also support customers requiring hydraulic products, hose assemblies, fittings and related equipment.
See our hydraulic trade counter locations.
Quick Hydraulic Oil Summary
PRIMARY JOB
Transfer hydraulic energy while helping lubricate and protect system components.
COMMON GRADES
ISO VG 32, ISO VG 46 and ISO VG 68 are widely encountered viscosity grades.
BEST SELECTION METHOD
Start with the machine manufacturer’s fluid specification.
DO NOT GUESS
The same viscosity grade does not necessarily mean two oils have identical specifications.
BEFORE MIXING
Identify the existing fluid and confirm compatibility.
Related Hydraulic Oil Guides
Need Help Choosing Hydraulic Oil?
If you are unsure about the viscosity grade or hydraulic oil specification required for your machinery, check the manufacturer’s information first or speak to our team for practical guidance.
Hydraulic Oil FAQs
What hydraulic oil should I use?
Use the viscosity grade and hydraulic-fluid specification recommended by the machinery manufacturer. Common grades include ISO VG 32, 46 and 68, but the correct oil depends on the equipment and operating conditions.
What is hydraulic oil used for?
Hydraulic oil transfers energy through a hydraulic system. It also helps lubricate components, carry heat and protect internal surfaces depending on the fluid formulation.
What does ISO 46 mean on hydraulic oil?
ISO VG 46 identifies a hydraulic oil viscosity grade. It describes the fluid’s viscosity classification and does not by itself define the complete additive package or performance specification.
Is ISO 32 thinner than ISO 46 hydraulic oil?
Yes. ISO VG 32 has a lower viscosity than ISO VG 46 at the standard reference temperature used for ISO viscosity classification.
Can you mix ISO 32 and ISO 46 hydraulic oil?
Mixing should not be assumed to be safe. The fluids may have different additive packages or specifications, and mixing them also changes the resulting viscosity. Compatibility should be confirmed first.
What happens if hydraulic oil is too thick?
Oil that is too viscous for the equipment and operating temperature can increase resistance to flow, affect cold operation and contribute to energy losses or heat generation.
What happens if hydraulic oil is too thin?
Oil with insufficient viscosity can reduce lubrication and increase internal leakage within pumps, valves and other hydraulic components.
Does hydraulic oil need to be kept clean?
Yes. Particles, water and degraded fluid can contribute to wear, corrosion and unreliable operation in pumps, valves, cylinders and other hydraulic components.
Does Completely Hydraulic supply hydraulic products in the UK?
Completely Hydraulic supports customers through its hydraulic trade counters and engineering network, supplying hydraulic products, hose assemblies, fittings and related support from locations serving London, Essex, Kent and Heathrow.