Hydraulic oil is essential for operating and protecting hydraulic systems by transmitting power, lubricating, and cooling machine components. The most common oils, labeled “32” and “46,” refer to the oil’s viscosity according to the ISO VG scale (International Standards Organization Viscosity Grade). Viscosity indicates how thick or fluid the oil is, where a higher number means a thicker oil that performs better at higher temperatures and under heavier loads.

The primary difference between hydraulic oil 32 and 46 is the viscosity, which affects their performance under different temperatures and pressures. Hydraulic oil 32 is best suited for systems operating in colder or more stable environments, while hydraulic oil 46 is better for warmer environments and higher loads. Choosing the correct oil for your system is important for optimizing efficiency and service life.

Hydraulic oil 32 is best suited for applications requiring fast and easy movement at low to medium temperatures. Its low viscosity reduces system resistance and contributes to energy-efficient operation.

Hydraulic oil 32 is used in many applications, primarily in workshop equipment and indoor environments where the temperature is stable and relatively low, such as production lines, industrial robots, and hydraulic lifts and platforms in controlled-temperature settings.

Hydraulic oil 46 is excellent for systems exposed to harsher conditions and higher temperatures. Its higher viscosity makes it thicker than hydraulic oil 32, providing better lubrication and protection under high temperatures and pressures. At low temperatures, the oil can become more viscous, making it best suited for warmer operating conditions. 

Hydraulic oil 46 is suitable for heavy machinery such as excavators, wheel loaders, and tractors used in demanding environments like construction sites, agriculture, and forestry. It is also suitable for industrial presses and heavier lift systems that operate under high loads and require reliable protection.

The viscosity index (VI) is a measure of how sensitive an oil’s viscosity is to temperature changes. It was developed to help compare the stability of oils under different temperature conditions.

A high VI means the oil’s viscosity changes less with temperature fluctuations, making it suitable for environments with significant temperature variations, such as outdoor applications. A low VI means the oil’s viscosity changes more with temperature shifts, making it better suited for systems where the temperature remains stable.

The suitability of an oil for specific applications depends on its VI, making it easier to select the right oil for different operating environments.

The table below shows the viscosity index (VI) and its impact on oil stability under temperature changes.

Using the wrong hydraulic oil could have negative consequences for your system.  Operational disturbances may occur, or you may face machine breakdown.  It is important to select the right hydraulic oil for your system, based on the specific working conditions. We encourage you to you carefully follow the manufacturers recommendations and monitor the viscosity of the oil to ensure problem-free operation. Below you will find a list of issues that may arise should you use the wrong hydraulic oil.

If you use an oil with too high viscosity, for instance hydraulic oil 46 in a system which requires 32, the oil may not be able to flow through. This increases your energy consumption and may lead to overheating.

If you use an oil with too low viscosity in a system which requires a thicker oil, for instance hydraulic oil 32 in stead of 46, you risk insufficient lubrication. This could lead to components wearing out faster, as the oil is unable to protect them at higher working temperatures or pressure.  

Using the wrong hydraulic oil can cause air bubbles to form in the system, a phenomenon known as cavitation. Cavitation can lead to severe component damage and significantly reduce the system's lifespan. Additionally, leaks may occur if the oil does not have the correct viscosity or fails to provide adequate sealing within the system.

Cavitation typically occurs when gas or vapor bubbles form in the oil in areas of low pressure. This can be caused by excessive suction height, an undersized suction line, high pump speed, or oil that is too thick. The phenomenon may also occur during cold starts when the oil is more viscous and flows more slowly.

When discussing excessive air in a hydraulic system, the term "free air" is often used, referring to trapped air pockets that can cause effects similar to cavitation. This issue is almost always due to mechanical problems, such as a leaking suction line or a low oil level in the reservoir. Free air can lead to what is known as the "diesel effect," where the system behaves like a diesel engine igniting due to compression heat. The diesel effect occurs when air trapped in the oil is subjected to rapid pressure increases, causing the temperature to rise to a point where the oil/air mixture ignites spontaneously. This can result in the formation of soot particles and additional heat generation, further degrading the system and potentially damaging components. 

Mixing different hydraulic oils should be avoided. If necessary, always consult your supplier to ensure compatibility. A hydraulic oil classified as ISO VG 46 from two different suppliers may contain different additives and base oils that are not chemically compatible. If one oil is based on a Group 1 base oil and the other on a Group 2 base oil, this can lead to sludge formation or varnish buildup, which may damage the system and reduce performance.

To keep your hydraulic system in optimal condition, it is essential to regularly monitor the oil’s condition and follow the recommended change intervals. Extending change intervals is possible, but only after a thorough analysis of the oil’s condition and if no signs of degradation are detected. Skipping oil changes can lead to increased wear, reduced performance, and, in the worst case, machine failure. Often, the need for an oil change is not noticed until it is too late.

It is also crucial to monitor the oil temperature. Oil that is too thin may indicate overheating or contamination, while overly thick oil could result from dirt buildup or low operating temperatures—both of which contribute to increased wear and decreased performance. Additionally, check the system for any leaks or issues with valves and pumps before refilling with new oil to prevent further complications after the change.

• Color change. Darker or discolored oil may indicate oxidation, contamination, or aging.

• Decreased performance. Slower or inefficient operation may result from a loss of lubrication properties or viscosity.

• Foaming or bubbles. Foamy oil or bubbles can indicate contamination or air ingestion, reducing efficiency.

• Unusual noises. Squeaking or rattling may signal inadequate lubrication or air pockets in the system.

• Higher operating temperatures. If the system overheats faster than usual, it could be due to thinning or contaminated oil, which compromises cooling and lubrication..

• Particles or sludge. Visible particles or sludge in the oil are clear signs that the oil has lost its purity and needs to be replaced.
  

Oil temperature directly impacts its lifespan. For mineral oils, oxidation occurs slowly up to 60°C, while synthetic esters can withstand up to 70°C. As a general rule, the oil’s lifespan is halved for every 10°C above 70°C. Therefore, maintaining system temperature is critical to extending oil life and preserving its lubrication properties.