Best Oil for Rotary Screw Air Compressors: ISO 46 or ISO 68?

INTRODUCTION

If you're new to rotary screw air compressors, you might be surprised at how important oil selection is. Just like picking the right engine oil for your car, using the correct oil ensures your compressor runs efficiently, stays cool, and lasts a long time. Two of the most common types are ISO 46 and ISO 68. But what do those numbers mean? And which one is right for your machine?

We'll explain everything step by step in plain, easy-to-understand language.

WHAT DOES "ISO" MEAN IN COMPRESSOR OIL?

ISO stands for the International Organization for Standardization, which sets standards for many products, including lubricants. The number (46 or 68) refers to the oil’s viscosity, or its thickness.

• ISO 46 is thinner.
• ISO 68 is thicker.

These numbers are part of the ISO VG (Viscosity Grade) scale, measured at 40°C (104°F), which is a typical operating temperature for compressors.

WHY VISCOSITY MATTERS FOR AIR COMPRESSORS

Viscosity affects how oil flows and protects moving parts. Here’s a simple analogy:

• Thin oil (like ISO 46) is like water — it flows easily.
• Thick oil (like ISO 68) is more like syrup — it moves slower and sticks better.

In a rotary screw air compressor, the oil needs to:

• Lubricate the rotors
• Cool the system
• Seal internal components
• Minimize wear and tear

Too thin, and it might not provide enough protection. Too thick, and it may cause resistance, overheating, or poor flow in colder conditions.

ISO 46 VS ISO 68: KEY DIFFERENCES AT A GLANCE

WHICH OIL SHOULD YOU USE IN YOUR ROTARY SCREW COMPRESSOR?

1. Check the Manufacturer’s Recommendation First

Always start here. Most rotary screw compressors have a recommended oil viscosity grade in the manual. This is based on the machine’s design and operating conditions.

2. Consider the Climate

• Cold or moderate environments (below 30°C / 86°F):

ISO 46 is usually better. It flows more easily on startup and reduces wear in colder temps.

• Hot or tropical environments (above 30°C / 86°F):

ISO 68 is preferred. It maintains viscosity under high heat and provides better protection.

3. Factor in the Application

Light-duty or intermittent use: ISO 46 may offer smoother starts and less energy consumption.

Heavy-duty, continuous operations: ISO 68 can handle heat better and offer longer protection.

4. Age and Condition of the Compressor

Older compressors with more wear may benefit from ISO 68’s thicker oil, which better fills the gaps and prevents leakage or metal-on-metal contact.

WHAT HAPPENS IF YOU USE THE WRONG OIL?

Using oil that’s too thin (like ISO 46 in a very hot climate) might result in:

• Overheating
• Increased wear
• Poor sealing

Using oil that’s too thick (like ISO 68 in cold weather) might cause:

• Hard starts
• Poor lubrication on startup
• Reduced efficiency

CAN YOU MIX ISO 46 AND ISO 68?

Generally, it’s not recommended to mix different oil viscosities. Doing so could:

• Create unpredictable flow characteristics
• Reduce the effectiveness of additives
• Shorten the lifespan of both oil and compressor

If you must change oil types, drain and flush the system thoroughly before switching.

SYNTHETIC VS MINERAL OIL (BONUS TIP)

Both ISO 46 and 68 oils come in mineral and synthetic varieties. Synthetic oils:

• Last longer
• Offer better protection at extreme temperatures
• Cost more upfront, but can save money in maintenance

If your compressor runs continuously or under harsh conditions, consider using a synthetic ISO 68 in hot climates or a synthetic ISO 46 in cooler ones.

CONCLUSION:

To sum it up:

Use ISO 46 in:

• Cooler climates
• Light to moderate-duty use
• Newer compressors

Use ISO 68 in:

• Hot environments
• Heavy-duty, continuous operation
• Older machines that need thicker oil

Always consult your compressor’s manual and consider climate, workload, and machine age when choosing between ISO 46 and ISO 68.