Can Engine Oil Be Used in Hydraulic Systems

[MikePhua]

Understanding the Purpose of Hydraulic and Engine Oils
Hydraulic oil and engine oil are both lubricants, but they serve fundamentally different roles in machinery. Hydraulic oil is designed to transmit power in hydraulic systems while also lubricating moving parts, reducing wear, and providing thermal stability. It is a non-compressible fluid that flows through pumps, valves, and cylinders to create motion and force. Engine oil, on the other hand, is formulated to lubricate internal combustion engines, reduce friction between metal surfaces, clean deposits, and manage heat generated during combustion.
The confusion between the two often arises because both oils share similar base stocks and viscosity ranges. However, their additive packages and operating environments are vastly different. Hydraulic oil typically lacks detergents and dispersants, which are essential in engine oil to manage soot, carbon buildup, and sludge. Engine oil is also designed to handle combustion byproducts, whereas hydraulic oil must remain clean and stable under high pressure.
Additive Packages and Compatibility
Engine oil contains a complex blend of additives:

• Detergents to clean internal surfaces
  
• Dispersants to suspend contaminants
  
• Anti-wear agents like zinc dialkyldithiophosphate (ZDDP)
  
• Viscosity modifiers for temperature stability
  
• Oxidation inhibitors and corrosion protectants
  

Hydraulic oil, by contrast, focuses on:

• Anti-wear agents for pump and valve protection
  
• Rust inhibitors to protect exposed metal surfaces
  
• Foam suppressants to prevent cavitation
  
• Seal compatibility additives to avoid swelling or degradation
  

Using engine oil in a hydraulic system may result in seal damage, reduced lubricity, and premature component wear. In high-pressure systems, the lack of proper anti-foam and oxidation resistance can lead to overheating and fluid breakdown.
A technician in Alberta once filled a hydraulic reservoir with 10W-30 engine oil during an emergency repair. While the machine ran temporarily, the pump began to whine within hours, and the control valves showed signs of sticking. After flushing and replacing with ISO 46 hydraulic fluid, performance returned to normal.
Manufacturer Exceptions and Historical Practices
Some manufacturers have historically recommended engine oil for hydraulic systems, especially in older or multi-purpose machines. For example:

• John Deere used 10W-30 or 15W-40 engine oil in hydrostatic systems for decades
  
• Case skid steers specified motor oil in hydraulic circuits to simplify service logistics
  
• Komatsu and New Holland have similar practices in certain models
  

These recommendations were often based on shared oil reservoirs, simplified inventory, and regional availability. However, modern hydraulic systems operate at higher pressures and tighter tolerances, making dedicated hydraulic fluids essential for long-term reliability.
A fleet manager in Texas standardized his equipment with ISO 68 hydraulic oil after experiencing seal failures in machines using multi-grade engine oil. The switch reduced downtime and improved component life across his loader fleet.
Viscosity and Temperature Considerations
Viscosity is a critical factor in oil selection. Engine oils are rated using SAE grades (e.g., 10W-30), which reflect cold-start and operating viscosity. Hydraulic oils use ISO grades (e.g., ISO 32, ISO 46), which are based on kinematic viscosity at 40°C.
Typical comparisons:

• SAE 10W-30 ≈ ISO 68
  
• SAE 15W-40 ≈ ISO 100
  
• SAE 5W-20 ≈ ISO 32
  

Using an oil with incorrect viscosity can lead to sluggish operation, poor lubrication, or overheating. Hydraulic systems require stable viscosity under pressure, while engines need flowability during cold starts and high-speed operation.
A restorer in Ontario used ISO 46 hydraulic oil in a compact excavator during winter. The machine struggled to respond until the fluid warmed up. Switching to ISO 32 improved cold-weather performance without sacrificing protection.
Contamination and Filtration Challenges
Engine oil is designed to suspend contaminants until they are removed by filters or drained. Hydraulic oil relies heavily on filtration and must remain clean to avoid damage to pumps, valves, and actuators. Mixing engine oil into hydraulic systems can overload filters and introduce sludge or varnish.
Best practices include:

• Never mix different oil types without manufacturer approval
  
• Flush systems thoroughly before switching oil types
  
• Use high-efficiency filters rated for the fluid in use
  
• Monitor fluid condition with regular sampling and analysis
  

A technician in Chile added a bypass filtration system to his hydraulic circuit after switching from engine oil. The added filtration captured residual sludge and extended fluid life by 40%.
Conclusion and Recommendations
While engine oil and hydraulic oil may appear similar, their roles, formulations, and operating environments are distinct. Substituting engine oil in hydraulic systems should only be done with manufacturer approval and under specific conditions.
Recommendations include:

• Follow OEM specifications for fluid type and viscosity
  
• Use dedicated hydraulic oil in high-pressure or precision systems
  
• Avoid mixing oils with different additive packages
  
• Monitor fluid condition and replace filters regularly
  
• Document any deviations from standard practice and observe system behavior
  

With proper fluid selection and maintenance, hydraulic systems can operate efficiently and reliably—delivering the power and control that heavy equipment demands.