What's the Difference Between Mineral, Synthetic and Biodegradable Hydraulic Fluids?

Hydraulic fluid does far more than transmit power. It lubricates moving components, carries heat away from critical parts, protects against corrosion and helps maintain the reliability of the entire hydraulic system.

Choosing the correct fluid has a direct impact on equipment performance, service intervals and operating costs. Mineral, synthetic and biodegradable hydraulic fluids all have their place, although each has been developed with different operating conditions and priorities in mind.

If you're replacing hydraulic oil, specifying fluid for new equipment or reviewing maintenance requirements, knowing the differences can help you make the right decision.

What Is Mineral Hydraulic Fluid?

Mineral hydraulic fluid is the most widely used type of hydraulic oil across manufacturing, construction, agriculture and mobile plant.

Produced from refined crude oil with carefully selected additives, mineral hydraulic fluids provide reliable lubrication, corrosion protection, oxidation resistance and wear protection for the majority of hydraulic systems.

Most equipment manufacturers specify mineral hydraulic oils unless the machine operates in particularly demanding environments or specific regulations require an alternative.

Typical applications include:

• Excavators and construction equipment

• Agricultural machinery

• Industrial hydraulic presses

• Forklift trucks

• Manufacturing equipment

• General hydraulic power packs

Advantages of Mineral Hydraulic Fluid

Mineral hydraulic fluids remain the industry standard for good reason.

They offer:

• Excellent value for everyday applications

• Wide compatibility with seals and hydraulic components

• Good wear protection

• Readily available viscosity grades and specifications

• Straightforward servicing and replacement

For many hydraulic systems operating under normal conditions, a quality mineral hydraulic oil is exactly what the manufacturer intended.

When Mineral Hydraulic Fluid May Not Be the Best Choice

Some operating environments place greater demands on hydraulic fluid than standard mineral oils are designed to handle.

Examples include:

• Very high operating temperatures

• Extremely low ambient temperatures

• Environmentally sensitive locations

• Equipment requiring extended drain intervals

In these situations, synthetic or biodegradable fluids may provide advantages.

What Is Synthetic Hydraulic Fluid?

Synthetic hydraulic fluids are formulated using chemically engineered base stocks rather than refined mineral oil.

The aim is to deliver more consistent performance across a wider operating temperature range while offering improved resistance to oxidation and thermal breakdown.

Although synthetic fluids generally carry a higher purchase price, they can reduce maintenance requirements in suitable applications and maintain performance under demanding operating conditions.

Typical industries include:

• Steel production

• Mining

• Marine applications

• Offshore equipment

• Heavy industrial processing

• Equipment operating in extreme temperatures

Advantages of Synthetic Hydraulic Fluid

Synthetic hydraulic fluids can provide:

• Excellent performance in both hot and cold environments

• Improved oxidation stability

• Longer fluid life

• Better resistance to thermal degradation

• Stable viscosity across changing temperatures

These characteristics help maintain consistent hydraulic performance where operating conditions are more demanding.

Are Mineral and Synthetic Hydraulic Fluids Interchangeable?

It depends on the equipment and the specification of the fluid already in the system.

Some synthetic hydraulic fluids are compatible with mineral oils, while others require the system to be drained and flushed before the new fluid is introduced. Mixing incompatible base oils or additive packages can reduce lubrication performance, affect seal compatibility and shorten the service life of the hydraulic system.

Before changing from mineral to synthetic hydraulic fluid, always check the equipment manufacturer's recommendations and confirm the compatibility of both fluids. If there's any uncertainty about the existing oil, draining the system and refilling it with the correct specification is usually the safest approach.

What Is Biodegradable Hydraulic Fluid?

Biodegradable hydraulic fluids are designed to reduce environmental impact if leaks or spills occur.

They are commonly used wherever hydraulic equipment operates close to waterways, forests, farmland or environmentally protected areas.

Several different formulations exist, including vegetable-based oils and modern synthetic ester fluids, each with different performance characteristics.

Common Applications for Biodegradable Hydraulic Fluids

Biodegradable hydraulic oils are frequently specified for:

• Forestry equipment

• Canal and river maintenance

• Marine and harbour operations

• Agricultural machinery

• Environmental management projects

• Local authority equipment

• Machinery working within protected habitats

Many contracts now specify biodegradable hydraulic fluids as part of environmental compliance requirements.

Advantages of Biodegradable Hydraulic Fluid

Depending on the formulation, biodegradable hydraulic fluids can offer:

• Reduced environmental impact

• Good lubricating performance

• Lower ecological risk following accidental leaks

• Compliance with environmental regulations and customer specifications

Modern biodegradable fluids have improved significantly over recent years and can provide performance comparable with conventional hydraulic oils in many applications.

Can Mineral, Synthetic and Biodegradable Hydraulic Fluids Be Mixed?

Mixing hydraulic fluids should never be treated as routine maintenance.

Different base oils and additive packages may react differently when combined, affecting lubrication performance, seal compatibility and overall system reliability.

Before topping up or changing hydraulic fluid, always check:

• Equipment manufacturer recommendations

• Fluid specification

• Viscosity grade

• Base oil compatibility

If the existing fluid cannot be identified with confidence, draining and refilling the system with the correct product is often the safest approach.

Which Hydraulic Fluid Is Right for Your Application?

There isn't a single hydraulic fluid that's suitable for every machine.

The correct choice depends on factors including:

Operating Environment

High temperatures, freezing conditions, dust, moisture and heavy-duty operation all influence fluid selection.

Environmental Requirements

Some industries require biodegradable hydraulic fluids to reduce environmental risk or meet contractual obligations.

Equipment Manufacturer Specifications

Manufacturer recommendations should always be the starting point. Hydraulic pumps, valves and seals are designed around specific fluid properties and viscosity grades.

Maintenance Strategy

Some operations prioritise maximum service life, while others focus on routine scheduled maintenance using readily available mineral hydraulic oils.

Why Does Hydraulic Fluid Viscosity Matter?

Viscosity determines how easily hydraulic fluid flows through the system. If the oil is too thin, it may not provide sufficient lubrication or maintain system pressure. If it's too thick, it can reduce efficiency, restrict flow and place additional strain on pumps and components.

That's why manufacturers specify viscosity grades such as ISO VG 32, ISO VG 46 and ISO VG 68. The correct grade depends on factors including operating temperature, equipment design and the demands of the application.

Using the specified viscosity helps maintain efficient hydraulic performance, protects components from unnecessary wear and supports reliable operation throughout the working life of the equipment.

Choosing the Right Hydraulic Fluid for Your Equipment

Selecting the correct hydraulic fluid helps protect components, maintain efficient system performance and support reliable operation throughout the service life of your equipment.

Choosing the right product comes down to following the manufacturer's specification, selecting the correct viscosity grade and considering the environment the machine operates in.

Shepherd Hydraulics supplies the complete OptimOil hydraulic fluid range, including mineral, synthetic and biodegradable hydraulic oils for industrial, agricultural, construction and mobile plant applications. As one of the UK's leading OptimOil distributors, we also offer flexible monthly and quarterly Subscribe & Save options for businesses with regular hydraulic oil requirements, helping to reduce administration and keep essential stock on hand.

If you're unsure which hydraulic fluid is suitable for your equipment, speak to the team at our trade counter or give us a call. Our specialists can help you identify the correct specification for your application and recommend the most suitable OptimOil product for the job.