Can engine oil be used as hydraulic oil?

Introduction

Imagine this scenario: you're working on a hydraulic machine (say, a small loader, excavator, or power steering system) and you find yourself short of the correct hydraulic fluid. You look around your shop and see a jug of engine oil sitting there and wonder: "Can I just use engine oil instead?"

This is a question many hobbyists, DIY mechanics, and even experienced technicians have asked. The short answer is: it's generally not recommended. While engine oil and hydraulic oil both lubricate moving parts, they are engineered for very different jobs-and using one in place of the other can lead to performance issues, damage, and safety risks.

In this post, we'll walk through:

• What engine oil and hydraulic oil are designed to do
• The key technical differences between them
• Whether, in theory, you could substitute engine oil
• What dangers and trade-offs that entails
• Safe practices, guidelines, and alternatives you can rely on

By the end, you'll have a clearer, practical understanding of why the answer to "Can engine oil be used as hydraulic oil?" is mostly no - and when you might (very temporarily) consider otherwise.

Let's start by clarifying exactly what engine oil and hydraulic oil are, and why they exist in two different categories.

What Is Engine Oil vs Hydraulic Oil

To understand whether engine oil can serve as hydraulic fluid, the first step is to grasp what each oil is made for, and how their roles differ in machines.

What Is Engine (Motor) Oil?

Engine oil, often called motor oil, is the fluid that lubricates, cools, and protects the internal parts of an internal combustion engine. Its main tasks include:

• Reducing friction between moving parts (bearings, pistons, camshafts)
• Carrying away heat (cooling)
• Cleaning (suspending contaminants, soot, by-products)
• Preventing corrosion and wear
• Sealing (helping form a thin film between rings/cylinders)

Because engines face high temperatures, pressures, combustion by-products, and dust, engine oils contain a rich additive package-detergents, dispersants, anti-wear agents, antioxidants, and more.

Engine oils are graded (e.g. 5W-30, 10W-40) per standards (such as SAE J300) to express their viscosity behavior under cold and hot conditions. 

What Is Hydraulic Oil?

Hydraulic oil (also called hydraulic fluid) is the medium used in hydraulic systems (e.g. excavators, jacks, hydraulic presses) to transmit power through pressure, while also providing lubrication and protection for parts.

Key functions of hydraulic oil:

• Transmit power: Under pressure, the fluid carries force from pumps to valves, cylinders, motors.
• Lubricate internal components: Pumps, valves, cylinders still need lubrication.
• Dissipate heat: The fluid absorbs and carries away heat generated by fluid friction.
• Control contaminants / water / foam: Hydraulic oil must manage air bubbles, water contamination, and keep the fluid stable.
• Protect surfaces / prevent corrosion

Because of its dual role (lubricant + power medium), hydraulic oil must satisfy stricter requirements: stable viscosity under pressure, good compressibility (bulk modulus), anti-foaming, good demulsibility (to separate water), seal compatibility, etc. 

How Their Roles and Requirements Differ

Here are the main points of difference, emphasizing why the two oils are not freely interchangeable:

Key Technical Differences

To judge whether engine oil might serve (even temporarily) as hydraulic fluid, it helps to see where they diverge at a technical level. Below are several critical properties and behaviors where engine oil and hydraulic oil differ-differences that often make substitution risky.

Viscosity & Viscosity Index (VI)

Viscosity index (VI) describes how well an oil resists thinning (losing viscosity) as temperature rises. Oils with a high VI change less with temperature.

Engine oils often use polymeric viscosity modifiers to boost the VI, so they behave "thicker" at high temps and "thinner" when cold.

However, those modifiers can be sensitive to mechanical stress (shear) and may degrade, causing the oil to lose viscosity over time. This is known as shear thinning or viscosity loss under shear. 

Hydraulic fluids are formulated to resist shear-induced viscosity loss, because a drop in viscosity under high shear leads to degraded performance. According to Mobil, maintaining shear stability is essential for hydraulic oils to preserve performance under a range of operating conditions. Mobil

Thus, even if you choose an engine oil with a matching SAE grade, its viscosity under working stress may deviate too far from what the hydraulic system expects.

Additive Chemistry (Anti-Wear, Anti-Foam, Oxidation, etc.)

Additive packages distinguish high-quality fluids from basic ones. Here are some differences:

• Anti-wear / extreme pressure (EP) agents: Hydraulic oil often contains additives that protect hydraulic pumps, valves, and cylinders under heavy loads. Engine oils also have anti-wear additives, but their formulation is optimized for piston bearings, cam/shuttle mechanisms, etc.
• Anti-foam agents: Hydraulic systems demand minimal foam and quick air release; foaming in a hydraulic circuit can cause cavitation, pressure fluctuations, or erratic responses. Engine oils may tolerate more foaming in an open crankcase environment.
• Oxidation stability: Because hydraulic systems often operate at moderate temperatures continuously, oxidation resistance is key. Engine oils have strong oxidation inhibitors, but the demands (and contaminant types) differ.
• Detergents / dispersants: Engine oil contains detergents and dispersants to suspend soot, combustion by-products, and other contaminants. Those are not needed (and can even be harmful) in hydraulic systems, where solid particles and water must often be filtered or separated, not held in stable suspension.
• Demulsifiers / water management: Hydraulic fluids often need to shed water (demulsify), so that water can separate and be drained; engine oils sometimes emulsify water, making separation harder.

Water Handling / Demulsibility & Contaminant Behavior

Hydraulic systems often encounter moisture ingress (humidity, condensation, leaks). It's crucial for hydraulic fluid to:

• Separate water from oil so the water can be drained or filtered out
• Avoid emulsifying water which can reduce lubrication, clog filters, or damage surfaces
• Minimize corrosion risk

Engine oils, by contrast, sometimes emulsify or hold contaminants in suspension, which is desirable for engines (to keep particles in solution until filtered) but undesirable in hydraulic systems where solid particles must be captured or settled, not circulated indefinitely.

Shear Stability & Permanent Viscosity Loss

As touched earlier, when fluid moves through tight clearances, orifices, or under high turbulence, it experiences shear forces. Good hydraulic fluid resists both temporary and permanent viscosity loss under shear.

Temporary shear thinning means the fluid gets a bit thinner while under stress, but recovers when stress is removed.

Permanent shear loss means the fluid's molecular structure (especially polymeric viscosity modifiers) breaks down, and the fluid cannot regain its original viscosity. 

Tests like Bosch Injector, Sonic Shear (ASTM D5621), and KRL (roller bearing test) are standard ways to gauge shear stability for hydraulic fluids. 

Engine oils often include VI additives which may not be as shear-resistant as those used in hydraulic fluids; over time, under continuous shear, they may degrade, causing the "effective grade" of the oil to fall below acceptable limits.

Seal / Material Compatibility & Chemical Effects

The elastomers, gaskets, hoses, and seals used in hydraulic systems are often formulated for specific fluid chemistries (for example, certain O-rings or seal materials are tested against hydraulic fluids).

Some additives or base oil components in engine oil may degrade or swell seals in hydraulic systems, causing leaks or premature failure.

Hydraulic systems may use seal materials not intended for the detergents, dispersants, or anti-corrosion agents typical in engine oils.

Theoretical Feasibility & Acceptable Scenarios

When might engine oil almost serve as hydraulic fluid? The answer is: only in very limited, low-risk cases-and with caution. In most practical systems, substitution leads to hidden damage over time.

When substitution might "work" (temporarily)

Low-pressure, low-load systems: Simple hydraulic circuits not under high force or precision demand - the mismatch is less immediately harmful.

Emergency or short-duration use: If your machine is stuck and you need to move it a little, a temporary substitution might suffice for minutes or hours.

Using hybrid / multi-purpose oils: Some machines are designed to use UTTO / multipurpose tractor oils that combine hydraulic, transmission, and wet-brake functionality. These are not pure engine oils. 

Viscosity match: If the engine oil you have closely matches the required viscosity of the hydraulic system, one major mismatch is reduced.

Why many substitutions fail over time

Shear degradation: Engine oils' viscosity modifiers may break down under continuous shear, causing the fluid to thin beyond acceptable limits.

Additive conflicts: Engine oil additives (detergents, dispersants) might interfere with hydraulic performance (foam control, water separation, seal compatibility).

Seal degradation & leakage: Some additives or base oils may chemically degrade seals in hydraulic systems, causing leaks and internal contamination.

Foaming, aeration, and cavitation: Air entrained in the fluid, or foam, can cause erratic pressure response, cavitation damage, or pump issues.

Performance drift: As fluid properties change, control response, efficiency, and consistency degrade.

Residues & flushing issues: After use, the leftover engine oil additives may resist flushing or leave deposits that hamper the correct hydraulic fluid later.

Guidelines if you absolutely do it anyway

Use only a clean, high-quality engine oil with a viscosity close to spec.

Operate only for a very short time, at low load, and watch system behavior (temperature, leaks, sluggish motion).

Be prepared to stop immediately if anomalies appear.

After the use, drain and flush the system completely, replace filters, inspect seals/hoses before refilling with correct hydraulic fluid.

Risks & Consequences

Using engine oil as hydraulic fluid (or mixing them) may seem convenient, but it carries a number of real risks and consequences. Over time, these issues compound and may lead to costly repairs or system failure.

Here are the main dangers:

Seal damage and leakage

One of the first problems is compatibility with seals, hoses, and gaskets:

• Additives or base oil components in engine oil may degrade or swell hydraulic system seals in ways their manufacturers didn't anticipate.
• Even if the system seems to run okay initially, over time the seals may become brittle, shrink, or leak.
• Leaks allow ingress of air and contaminants, compounding system problems.

Accelerated wear, fatigue & component damage

Because engine oil is not optimized for the shear loads, pressures, and flow paths in hydraulic systems:

• The lubrication film may not be adequate under high-pressure zones, leading to metal-to-metal contact, wear, scoring, or fatigue.
• Valves, pump parts, pistons, and cylinder surfaces can incur additional stress or uneven wear.
• Components designed for hydraulic oil's specific additives (e.g. anti-wear, EP agents) may not be protected as intended under engine oil chemistry.

Filtration & blockage issues

Engine oil's additive packages (dispersants, detergents) may keep particles or water in suspension, making filtration less effective.

Suspended contaminants may clog fine hydraulic orifices, valve ports, or filters.

If water is emulsified in the fluid, it may lead to micro-corrosion or reduce lubrication quality.

Viscosity drift, efficiency loss & sluggish response

Under shear and over time, engine oil's viscosity modifiers may degrade (shear thinning or permanent viscosity loss).

The fluid may become too thin (losing load-bearing capability) or too thick (increasing pump load).

As fluid behavior becomes less predictable, hydraulic response (actuation speed, control precision) worsens.

System efficiency drops, raising internal heating and possibly aggravating thermal stress.

Air entrainment, foaming, and cavitation

Hydraulic systems require prompt air release and minimal foaming to maintain stable operation.

Engine oils may tolerate more air or foam in lubrication circuits, but in hydraulics these can cause erratic pressure response, cavitation, or pump damage.

Bubbles or foam can collapse under pressure, causing noise, vibration, or pitting.

Safety, overheating, reliability & maintenance cost

As system efficiency drops or leaks start, fluid temperatures may rise beyond design limits, accelerating oxidation and further degrading the fluid.

Unexpected failures (burst hoses, pump seizure, cylinder malfunction) can create safety hazards.

Frequent maintenance, fluid changes, and component overhauls may become necessary.

Using the wrong fluid might void warranties or violate manufacturer guidelines. (Many expert sources warn it's unsafe to substitute engine oil for hydraulic fluid.) 

Real anecdote / cautionary tale

In online forums, users have shared cautionary stories:

• "Reward = save $400 in hydraulic oil. Risk = $20,000 transmission lost, probably at the worst possible time. I would drain and save to use in a dump…" 
• Another user admitted mistakenly adding engine oil to a hydraulic reservoir and, although the machine ran briefly, others strongly recommended changing the fluid before more damage occurred.

Practical Tips & Best Practices (Including Substitute Evaluation)

Here's a combined, more compact guide that includes both substitute evaluation and general operational best practices.

Stick with OEM / Specified Hydraulic Fluid Whenever Possible

Always use the type, viscosity grade, and additive class that the equipment manufacturer recommends.

Avoid mixing fluids unless confirmed compatible.

 How to Judge a Substitute if You Must Use One

If you ever evaluate using engine oil or another nonstandard fluid:

• Compare viscosity at operating temperature - the closer to the hydraulic spec, the safer.
• Check shear stability / viscosity index - the fluid must resist thinning under stress.
• Verify seal compatibility - ensure the fluid will not degrade or shrink seals.
• Confirm foam and air release behavior - the fluid should shed air quickly and resist foaming.
• Ensure water separation / demulsibility - water contamination must be separable, not held in suspension.
• Look at oxidation and thermal stability - the fluid should survive expected temperature ranges without breaking down.
• Test the substitute in a low-risk area first (low pressure, low load) before full use.

Operational & Maintenance Best Practices

Cleanliness is critical: prevent dirt, dust, water contaminating your fluid. Use filtered oil when topping up.

Monitor the system: watch temperature, pressure, unusual noises, leaks, sluggish response, foaming.

Maintain filters & seals: replace filters on schedule, inspect hoses, seals, fittings regularly.

Flush after substitute use: if you ever used engine oil in place of hydraulic fluid, fully drain, flush lines, replace filters/parts, then refill with correct fluid.

Use multi-purpose tractor fluids (with care): In some tractor or agricultural equipment applications, UTTO (Universal Tractor Transmission Oil) or similar multipurpose oils are used to serve hydraulic, transmission, and brake systems. These are formulated to balance competing needs and are different from pure engine oils. 

Limit substitution duration: If forced to use a substitute, keep the runtime as short as possible and return to correct fluid at the earliest opportunity.

Conclusion & FAQs

Conclusion

To wrap up: although engine oil and hydraulic oil may appear similar at first glance, they are engineered for very different jobs. Hydraulic fluid isn't just a lubricant - it bears pressure, transmits force, and must behave reliably under shear, temperature changes, and contaminant exposure. Substituting engine oil for hydraulic oil is fraught with risks: seal damage, increased wear, viscosity drift, foaming, leaks, and loss of control responsiveness.

That said, in rare emergency scenarios and low-stress systems, a carefully chosen engine oil might be used temporarily - but only with strong caution, close monitoring, and a plan to flush and restore the correct fluid immediately.

Using the steps described in earlier sections (comparing properties, bench testing, close observation, system flushing), you can reduce damage risk if substitution is absolutely unavoidable. But the best practice is always: stick with the OEM-recommended hydraulic fluid.

FAQs

Q1: What happens if I mix engine oil with hydraulic fluid?
A: Mixing increases unpredictability. Additive packages may conflict, contaminant behavior may change, seal compatibility may worsen, and performance may degrade. In many cases, the mixture is worse than either oil alone.

Q2: Can hydraulic oil be used in an engine?
A: Not safely. Hydraulic oils lack necessary detergents, dispersants, and protections for combustion by-products. Using hydraulic fluid in an engine may lead to deposit formation, wear, and engine damage.

Q3: How long can I run a hydraulic system on engine oil in an emergency?
A: As short as possible - ideally just minutes or hours to move equipment to safety or maintenance. Any longer use greatly increases the risk of damage. Replacement should follow immediately.

Q4: How can I tell if using engine oil damaged my hydraulic system?
A: Look for signs like increased leaks, worn seals, rising temperature, foaming, erratic operation, inability to hold pressure, or unusual noises. You can also perform fluid analysis (particle counts, wear metals).

Q5: Are there oils designed for both engine and hydraulic use?
A: Yes - so-called "multi-purpose" or "universal tractor transmission/hydraulic oils" (UTTO / STOU / hydraulically compatible oils). These are specially formulated to balance engine and hydraulic requirements. But be sure they match your system specifications before using them.

A Note About Poocca

If you ever need high-quality hydraulic components (pumps, motors, valves, systems) to pair with proper fluid practice, Poocca Hydraulic is one supplier worth considering. As a hydraulic manufacturer based in China, Poocca offers over 1,600 types of hydraulic products - gear pumps, piston pumps, vane pumps, hydraulic motors, valves, and more - with ODM / OEM support.