In a hydraulic system, the oil is the working medium. Pressure applied at one point in the circuit is transmitted through it to move cylinders, motors and actuators elsewhere in the system. Alongside that, it lubricates internal components, helps seal fine internal clearances, and carries heat away from parts that would otherwise run too hot.
This is why substituting any oil of roughly the right thickness causes problems that general industrial lubricants wouldn’t solve. Hydraulics have to meet all of these jobs at once, not just resist wear, which is why dedicated hydraulic oil formulations exist for industrial machinery, rather than treating them as interchangeable with other oils used across industrial systems.
Hydraulic oils are specified using ISO viscosity grades, most commonly ISO VG 32, 46 and 68, with the number referring to the oil’s viscosity at 40°C. Choosing the correct grade for a given hydraulic system depends on pump type, operating pressure and the operating temperatures the equipment runs in, and it’s specified by the OEM in the equipment documentation rather than something to estimate on site.
Running an oil that’s too thin increases internal leakage and wear on pump components under high pressure, while one that’s too thick increases pumping resistance and can cause cavitation on cold start-up. ISO 46 is one of the most common grades specified for general industrial hydraulic systems, and getting the viscosity grade right is the first decision in choosing a hydraulic oil, before brand or fluid type even comes into it.
For most general hydraulic systems used across industrial machinery, a standard rust and oxidation inhibited (R&O) or anti-wear (AW) mineral oil is the correct starting point. These formulations protect against corrosion inside the system, resist oxidation over extended service life, and include anti-wear additives to protect pump and valve components under load. Texaco Rando WM 32 is a multigrade hydraulic oil of this type, formulated for reliable performance across a wide range of operating conditions, and suited to a broad spread of industrial machines, construction equipment and mobile equipment beyond just one specific application.
Where equipment operates in or near environmentally sensitive locations, standard mineral oil carries a real risk if a leak or spillage occurs. Biodegradable hydraulic oils are formulated to break down far more readily in the environment than mineral oil, while still providing the anti-wear protection a hydraulic system needs. Total Biohydran TMP 32 is designed for exactly this situation, recommended for use where incidental spillage in environmentally sensitive areas is a genuine possibility, such as forestry work, water catchment areas or hydroelectric installations. It’s worth considering for any hydraulic machinery operating outdoors or near watercourses, not just equipment already flagged as an environmental risk.
Certain applications carry a fire risk if oil escapes near a heat source, such as hot machinery, furnaces or steam plant. For these, a fire-resistant formulation based on phosphate esters is specified instead of mineral oil, since it resists ignition even if sprayed or leaked near a high-temperature surface. Total Hydransafe FR EHC is this type of product, designed for hydraulic circuits requiring safety fluids, including electro-hydraulic governor systems on steam turbines. This is a specialist category, and equipment specified for it shouldn’t be run on standard mineral oil as a substitute, since seal and material compatibility requirements differ between the two.
Switching hydraulic oil type, particularly moving to or from a phosphate ester product, isn’t always a simple drain-and-refill. Seal materials, hoses and some internal components can be compatible with one oil family but not another, so a system flush and compatibility check is worth confirming before changing type on existing equipment.
This is also different from automatic transmission fluid, which is specified separately for vehicle gearboxes rather than industrial hydraulic systems, so the two shouldn’t be assumed interchangeable even where viscosity looks similar on paper.
Water contamination is another common cause of premature failure, since even small amounts of water reduce an oil’s lubricating film strength and accelerate component wear. Regular oil analysis and proper filtration during top-ups help catch contamination before it causes damage, rather than only discovering a problem once a pump has already failed.
Hydraulic oil is one of several distinct lubricant categories used across industrial equipment, each suited to a different job inside the same machine. Choosing the right hydraulic oil ultimately comes down to matching viscosity grade and oil type to the system’s requirements, in the same way our guide to different types of lubricants and their uses covers how hydraulic oils compare to gear oils, greases and engine oils more broadly.
If you’re unsure which hydraulic oil is right for your equipment, our team can help. Contact Velocity Edge and a knowledgeable member of our team can assist you in selecting the correct product for your application.