Understanding Hydraulic Fluid Types: Which One Does Your System Need?

Choosing the wrong hydraulic fluid for your equipment is like putting diesel in a petrol engine—it simply won't work properly and can cause expensive damage. Yet many UK businesses use whatever fluid they find or what's cheapest, rather than following manufacturer specifications.

ISO Viscosity Grades Explained

Hydraulic fluids are classified by viscosity using ISO grades, typically ranging from ISO 10 to ISO 68. The number indicates the fluid's thickness at 40 degrees Celsius. Lower numbers flow more easily in cold conditions, whilst higher numbers provide better film strength in high-temperature applications. Most industrial equipment uses ISO 46, which is a good middle ground for UK temperatures and typical operating conditions.

Mineral Oil Fluids

Traditional mineral-based hydraulic fluids are affordable and work well in most standard applications. They're compatible with most seals and components manufactured before the 2000s. However, they're less environmentally friendly and have lower fire resistance than synthetic alternatives.

Fire-Resistant Fluids

If your equipment operates near high temperatures or flammable materials, fire-resistant fluids are essential. These include water-glycol and phosphate ester fluids. They're more expensive but significantly reduce fire risk in hazardous environments. They're mandatory in certain industries and for specific equipment types.

Synthetic and Biodegradable Options

Synthetic fluids offer superior performance across wider temperature ranges and last longer than mineral oils, reducing replacement frequency. Biodegradable hydraulic fluids are increasingly popular for environmental compliance, particularly for equipment used near waterways or sensitive areas. They perform well but cost more and may require component upgrades.

Additives and Performance Boosters

Modern hydraulic fluids contain additives that improve performance. Anti-wear additives protect pumps and motors. Anti-foam agents prevent aeration. Corrosion inhibitors protect against rust. Thermal stability improvers help the fluid maintain properties across temperature ranges. Always use fluids with appropriate additive packages for your system type.

Mixing Fluids: A Critical Warning

Never mix different hydraulic fluid types. Incompatible fluids can react chemically, causing seal damage, sludge formation, and system failure. If you need to change fluid types, fully flush the system with the new fluid specification first.

Finding Your Specification

Your equipment manual specifies the exact fluid needed. If you've lost documentation, contact the manufacturer with your equipment model number. Quick Hydraulics can help you identify the correct specification and source quality fluids for your system.