Hydraulic systems function by transmitting energy and power through the movement of fluids. The fluids used in these systems — referred to as hydraulic fluids or hydraulic oils — play a vital role across countless types of machinery and industrial equipment. Hydraulic systems and components appear throughout agriculture, construction, mining, steel production, shipbuilding, aviation, aerospace, chemicals, medicine, food processing, environmental technology, and many other fields. It is virtually impossible to name an industry that does not rely on some form of fluid technology, whether in stationary or mobile applications.
The advantages of hydraulics and hydraulic oil are numerous. Among the most significant is that they can serve as a more environmentally responsible choice in certain scenarios. Beyond this, these fluids are capable of bearing substantial loads. Their anti-wear, anti-corrosion, and other protective properties allow them to safeguard hydraulic systems even in the most challenging operating environments.
What exactly is hydraulic oil, and in what contexts is it used? What additional advantages do high-performance hydraulic oils offer to equipment operators and owners? The Valvoline™ team has compiled answers to some of the most frequently asked questions surrounding hydraulic oils.
Hydraulic oil is a multifunctional fluid. Its roles include acting as a medium for energy transfer and power transmission, functioning as a lubricant, and serving as a sealant. It also dissipates heat from the equipment and carries away contaminants.
Because hydraulics can be divided into hydrodynamics and hydrostatics, distinct types of hydraulic fluids exist for each. Fluids designed for hydrodynamic applications are categorized as power-transmission oils, while those intended for hydrostatic use are called hydraulic oils. In hydrostatic applications specifically, operating pressures are elevated and flow rates are relatively low. As a result, hydraulic oils must be non-compressible fluids capable of transmitting power within a system or piece of equipment.
Among all lubricating fluids, hydraulic oils rank as the second most critical category after engine oils. Since hydraulics fall into three major domains — stationary, mobile, and aviation — each area demands a hydraulic medium suited to its particular operating requirements. When selected and used correctly, hydraulic oils conserve energy, minimize wear on machines and components, extend maintenance intervals, and prolong overall machine life.
Like most lubricants, hydraulic oils are primarily classified as either mineral-based or synthetic. Mineral hydraulic oils originate from petroleum, whereas synthetic varieties are chemically engineered. Beyond these two main categories, the classification also includes fire-resistant hydraulic oils for use in hazardous environments, biodegradable and food-grade options, STOU and UTTO universal mobile hydraulic oils, and fluids specifically designed for aircraft hydraulic systems.
As noted earlier, hydraulic systems are encountered in virtually every industry. An extensive variety of hydraulic machinery, equipment, and components depend on hydraulics and require hydraulic oil to sustain their operation.
Selecting the appropriate hydraulic oil and determining the right specifications involves evaluating several key factors: the design and type of hydraulic system and pump, the expected temperature and pressure ranges during operation, and any relevant environmental considerations.
Mineral-based hydraulic oils are produced from crude oil fractions that undergo refining until they achieve suitable lubrication characteristics. They are then fortified with a carefully selected set of additives. These additives serve a variety of purposes, including anti-wear protection, rust and oxidation inhibition, viscosity index improvement, and more.
Because they are petroleum-derived, mineral hydraulic oils are a more economical option relative to synthetic alternatives. Depending on the quality of the base oil and additive formulation, these mineral lubricating hydraulic oils can deliver strong performance — as demonstrated by the Valvoline HVLP and HLP conventional hydraulic oils. The additive packages in mineral-based hydraulic oils defend hydraulic systems against corrosion, wear, and water contamination. They also enhance the oil's demulsifying characteristics and improve the viscosity index to better withstand temperature fluctuations. Hydraulic oils with detergent and dispersant properties can also be found within this category.
Synthetic hydraulic oils were developed to address the limitations inherent in mineral-based options. Formulated using chemically engineered base oils, they deliver superior performance compared to their mineral counterparts. Their advantages include proven reliability at elevated temperatures, outstanding oxidation stability, and biodegradability. That said, synthetic fluids come at a considerably higher price, may exhibit significant toxicity, and could be incompatible with certain seal materials.
Fire-resistant hydraulic oils are formulated using both mineral and synthetic base oils that biodegrade at a rapid rate. As their name suggests, these hydraulic fluid types are designed for environments where fire risk is elevated, such as aviation, mining, steel mills, and die-casting operations. This critical fire-resistant property makes them applicable across a broad range of uses, and they are generally less costly than synthetic hydraulic oils. Their anti-wear performance, however, falls short of synthetic fluids. Nevertheless, the additive systems incorporated into fire-resistant hydraulic oils enhance corrosion protection, reduce friction, and limit foaming.
Fire-resistant hydraulic oil can take several forms: mineral oil-based oil-in-water emulsions, synthetic water-based solutions, mineral oil-based water-in-oil emulsions, water-based formulations, and water-free versions.
This class of hydraulic oils emerged in response to increasingly stringent environmental protection standards. These oils offer ecological compatibility and are suitable for both stationary and mobile use across industrial, forestry, mining, tunnel construction, and earthmoving applications.
In recent years, biodegradable hydraulic oils — such as the Valvoline Ultramax Bio-46 premium synthetic hydraulic fluid — have been progressively displacing mineral oil-based hydraulic lubricants. These oils can be further broken down into several subgroups including:
Given their properties, biodegradable hydraulic oils are also well-suited to marine environments and the hydraulic systems of boats and vessels.
This category is governed by a range of international guidelines that specify which base oils and additives may be used in these hydraulic fluids. Food-grade hydraulic oils are divided into subgroups that distinguish between applications where no direct food contact is possible and those where incidental food contact may occur. The base oils used in food-grade hydraulic lubricants consist of special white oils, select polyalphaolefin grades, and particular polyglycols.
Universal tractor transmission oil (UTTO) and super tractor universal oil (STOU) are hydraulic gear oils and fluids designed for the gearboxes and hydraulic systems of agricultural machinery and tractors, accommodating both wet-brake and non-wet-brake configurations.
These oils are formulated for the hydraulic systems found in aircraft, where they must withstand high pressures and extreme climatic variations involving dramatic temperature swings. Consequently, these fluids must demonstrate exceptional thermal stability and maintain a high viscosity index. They are also required to be fire-resistant and completely free of contaminants.
The lubricating performance of hydraulic oils is achieved through the incorporation of additives. These additive systems — also referred to as additive packages, additive mixtures, or add packs — each carry distinct properties designed to fulfill specific functions. Developing an additive package is a highly precise undertaking, as each additive must be compatible with the others and work in concert with them.
Key base oil properties that additives can enhance include stability, anti-corrosion performance, anti-wear protection, viscosity-temperature behavior, foam resistance, detergency, water separation capability, and friction coefficient — among the most critical attributes.
Additives can be organized into two fundamental categories based on their function:
A further classification system, based on the additive formulation used in the hydraulic oil, is organized as follows:
The categorization above illustrates that hydraulic oils must meet a defined set of performance criteria to satisfy the demands of hydraulic systems and working conditions. Whether the application is heavy-duty or operates under less extreme conditions, hydraulic oil is expected to be:
Beyond performance, the hydraulic fluid selected must also represent a cost-effective solution for the operator or owner of the hydraulic equipment.
It is rarely possible for a single hydraulic oil to satisfy every requirement listed above, which is precisely why hydraulic oils are custom-formulated to match the specific conditions of their intended application.
Viscosity is one of the most critical characteristics of hydraulic oils, reflecting the fluid's ability to resist changes caused by temperature variation. The viscosity index of mineral-based hydraulic oils can be raised through the use of additives. Hydraulic oils with a high viscosity index, for example, enhance the performance of hydraulic systems and pumps at low temperatures while simultaneously offering wear protection at high temperatures.
The density of hydraulic oil plays a significant role in the lubrication and overall performance of hydraulic machinery. Most systems are designed to pump fluids within a specific density range, meaning that any shift in hydraulic oil density will affect the pump's operating efficiency. When fluid density rises, for instance, so does its erosive potential. In high-turbulence systems, the hydraulic fluid can degrade valves, piping, and other components and surfaces it contacts.
Conversely, hydraulic oil with higher density can improve contamination control by aiding in the suspension, transport, and removal of contaminants. In such cases, particles remain suspended longer, allowing filters to capture them more effectively and making the overall system easier to maintain.
Every hydraulic oil has its own distinct characteristics, and the benefits it provides will vary based on its target application and the composition of its additive package.
In general, hydraulic oils with either mineral or synthetic base oils are capable of delivering the following benefits:
Hydraulic systems that leverage the power of fluid are found across an extraordinarily wide array of machinery and components — present in nearly every industry and designed to function under diverse working and environmental conditions. The hydraulic fluids used within these systems can be grouped into several categories according to their formulation and intended use. They serve simultaneously as energy-transfer mediums, lubricants, coolants, sealants, and agents for removing various contaminants from the system.
From its earliest days, Valvoline™ has remained a pioneer in lubrication and additive technology. Drawing on more than 150 years of practical experience and continuous innovation, Valvoline consistently delivers lubricating products of the highest caliber. Working alongside equipment manufacturers, Valvoline provides premium hydraulic lubricants that support operations across numerous industries while conforming to international standards and specifications. The Valvoline hydraulic oil lineup encompasses conventional mineral oil-based and premium synthetic options, HVLP and HLP grades, and biodegradable hydraulic oils formulated for environmentally sensitive applications.
