Why ISO 46 Hydraulic Oil Should Never Replace Rotary Screw Compressor Oil

If you own or maintain a rotary screw air compressor, you already know that the heart of the machine is its oil. And while ISO 46 hydraulic oil sits on the same shelf as compressor oil- and sometimes even shares a similar viscosity- the two products are not interchangeable.

In fact, using the wrong oil can slowly damage your compressor, shorten its life, and lead to expensive repairs that no business wants to deal with.

Here is a clear, no-nonsense explanation that will help you understand the differences and protect your equipment.

WHAT ROTARY SCREW COMPRESSOR OIL IS DESIGNED TO DO?

Rotary screw compressors run hot, fast, and continuously. Because of this, their oil has to perform several heavy-duty tasks at the same time:

• Lubricate the rotors and bearings under high temperature and pressure
• Seal the compression chamber for efficient airflow
• Control varnish and sludge that form at high heat
• Manage moisture that naturally occurs inside compressed air systems
• Maintain viscosity stability even during long hours of operation

This combination of requirements is very specific, which is why compressor oil contains additives like anti-oxidants, detergents, demulsifiers, and anti-foaming agents that match the needs of rotary screw machines.

WHAT ISO 46 HYDRAULIC OIL IS DESIGNED FOR?

Hydraulic oil has a completely different job. It is formulated for systems where pressure transfers force- like cylinders, pumps, and valves.
Hydraulic oil focuses on:

• Stable pressure transfer
• Basic lubrication
• Anti-wear protection
• Water tolerance and rust prevention

Hydraulic oil is NOT built to handle extreme, continuous heat or compressive environments. And that’s where problems start.

WHY YOU SHOULD NOT INTERCHANGE ISO 46 HYDRAULIC OIL WITH COMPRESSOR OIL?

1. Overheating and Varnish Build-Up

Rotary screw compressors run hotter than hydraulic systems. Hydraulic oil will start breaking down much sooner, forming:

• Varnish
• Sludge
• Carbon deposits

These deposits coat the air end, bearings, and valves, making the compressor work harder until it eventually overheats or fails.

2. Poor Moisture Control

Compressor oil is formulated to separate water quickly so it can be removed through the condensate system.
Hydraulic oil tends to hold water, leading to:

• Rust
• Accelerated wear
• Contaminated compressed air

This is a common reason compressors lose efficiency over time.

3. Loss of Lubrication Under High Heat

Hydraulic oil does not maintain viscosity as well inside a compressor.
As the oil thins out, bearings and rotors lose proper lubrication, which leads to metal-to-metal contact- a fast path to air-end destruction.

4. Foaming and Oil Carryover

Hydraulic oil foams more easily inside the compression chamber. This pushes oil downstream into:

• Filters
• Dryers
• Air tools
• Production equipment

Not only does this increase operating costs, but it also contaminates your entire air system.

5. Warranty and Service Issues

Manufacturers clearly specify compressor-grade lubricants. If hydraulic oil is used- even accidentally- it can:

• Void warranty coverage
• Complicate service claims
• Require costly system flushing and component replacement

It is simply not worth the risk.

THE BOTTOM LINE

Both oils may share “ISO 46” on the label, but the similarity ends there.
Rotary screw compressors are engineered for continuous, high-temperature operation, and they need a lubricant built specifically for that environment. Hydraulic oil cannot perform the same job and can easily cause irreversible damage.

If you operate compressors in the U.S. or Canada- especially in demanding manufacturing, automotive, or construction environments- using the correct compressor oil is one of the simplest and most cost-effective ways to maintain long-term reliability.

LOOKING FOR GUIDANCE ON COMPRESSOR OIL OR MAINTENANCE?

At MasterAire, we support businesses across North America with reliable rotary screw compressor solutions, maintenance guidance, and technical support. If you’re unsure which oil your machine should use or need help preventing premature wear, feel free to reach out- we’re always happy to point you in the right direction.