Choosing the correct lubricant is one of the most important factors in protecting equipment and maintaining reliable operation. Whether you’re maintaining industrial gearboxes, hydraulic systems, or fleet vehicles, understanding oil viscosity is essential for ensuring machinery performs as intended.

One challenge many technicians and maintenance managers encounter is that viscosity can be expressed in multiple grading systems, including ISO VG, AGMA, and SAE. Because these standards were developed for different industries, converting between them isn’t always intuitive.

This guide explains how oil viscosity works, why multiple grading systems exist, and how to translate between them using a viscosity conversion chart. We’ll also highlight the types of lubricants commonly used within each viscosity range so you can identify the right product for your equipment.

Oil viscosity describes a lubricant’s resistance to flow. In practical terms, viscosity determines how easily an oil moves and how effectively it forms a protective film between moving components.

Thinner oils with lower viscosity flow more easily and are often used in systems that require quick circulation, such as hydraulic equipment. Thicker oils with higher viscosity flow more slowly and are designed to maintain a protective film under heavier loads or slower-moving mechanical systems.

Selecting the correct viscosity is critical because it directly affects lubrication performance. If the oil is too thin, it may fail to maintain a protective film, leading to increased wear. If it is too thick, it may not circulate properly, which can cause overheating or reduced efficiency.

Different types of equipment are designed to operate within specific viscosity ranges. For example, many hydraulic systems use oils in the ISO VG 32 to ISO VG 46 range, while industrial gearboxes often require heavier lubricants such as ISO VG 220 or ISO VG 460.

At Keller-Heartt, these viscosity ranges are commonly found across several lubricant categories, including industrial gear oils, hydraulic oils, compressor oils, and food-grade lubricants used in processing environments. Understanding viscosity ratings helps ensure you select the appropriate lubricant for each application.

Because lubrication requirements vary widely between industries, different organizations developed viscosity classification systems tailored to their equipment and operating conditions.

The most widely used standards today include the following.

Each system measures viscosity differently or defines grades based on performance requirements for specific equipment types.

The ISO viscosity grading system, developed by the International Organization for Standardization, was designed primarily for industrial lubrication. It classifies oils according to their kinematic viscosity at 40°C, providing a simple and consistent way to compare lubricants used in manufacturing equipment.

The AGMA system, created by the American Gear Manufacturers Association, focuses specifically on industrial gear protection. These grades correspond to viscosity ranges that perform well under the heavy loads and sliding contact typical of gear systems.

The SAE scale, developed by the Society of Automotive Engineers, serves the automotive industry. Separate grading systems exist for engine oils and gear oils, which is why the numbers used in automotive gear lubricants may appear much higher than those used in engine oils.

Because equipment manufacturers reference different systems, technicians often rely on viscosity conversion charts to identify equivalent oils when selecting or replacing lubricants.

ISO VG, or ISO Viscosity Grade, is the most widely used classification system for industrial lubricants. The number associated with each grade represents the oil’s approximate kinematic viscosity at 40°C, measured in centistokes (cSt).

For example, an oil labeled ISO VG 220 has a viscosity close to 220 centistokes at 40°C.

The ISO grading system covers a wide range of viscosities used in industrial applications.

These oils are commonly used throughout manufacturing environments, where proper lubrication ensures equipment reliability and reduces downtime.

Many industrial lubricants supplied by Keller-Heartt follow ISO viscosity standards. Products such as industrial gear oils, compressor oils, hydraulic fluids, and food-grade lubricants are typically formulated within specific ISO VG ranges to meet equipment requirements.

Understanding ISO viscosity grades is often the first step in selecting the correct lubricant for industrial machinery.

While ISO VG provides a universal standard for industrial lubricants, the AGMA grading system focuses specifically on gear lubrication. Gear systems operate under heavy loads and significant sliding contact, which requires lubricants capable of maintaining a stable film between gear teeth.

AGMA grades correspond to viscosity ranges that provide effective protection for these demanding conditions.

Industrial gearboxes used in manufacturing, mining, material handling, and processing equipment frequently rely on these viscosity ranges.

For example, AGMA 5 (approximately ISO VG 220) is a common choice for industrial gear drives operating under moderate to heavy loads. Higher grades such as AGMA 6 or AGMA 7 are often used in larger gearboxes where higher film strength is required.

Keller-Heartt supplies gear lubricants designed to meet these specifications, including products used in industrial gear drives, conveyors, and heavy machinery.

The SAE viscosity classification system is widely used in the automotive industry, but it includes two separate grading systems.

One of the most common sources of confusion is that SAE gear oil numbers appear much higher than engine oil numbers, even when the viscosity is similar.

For example, SAE 90 gear oil is roughly equivalent in viscosity to ISO VG 220, even though the numbers look very different.

This difference exists because the SAE gear oil scale was developed independently from the engine oil classification system.

SAE gear oils are commonly used in applications such as:

• Automotive differentials
• Truck transmissions
• Fleet maintenance equipment
  

Keller-Heartt supplies lubricants used in many of these applications, particularly for fleet maintenance, automotive service, and heavy-duty machinery.

Because different equipment manufacturers specify different standards, technicians frequently need to convert between viscosity systems.

A viscosity conversion chart provides a quick way to identify approximate equivalents between ISO, AGMA, and SAE grades.

These conversions are particularly useful when:

• Replacing lubricants in existing equipment
• Cross-referencing manufacturer recommendations
• Identifying equivalent products between suppliers

Using a chart like the one above helps ensure technicians select lubricants that maintain the proper viscosity range required for reliable operation.

If you frequently work with industrial lubricants, having a quick-reference viscosity conversion chart can make product selection much easier.

The full chart compares several common viscosity systems, including:

• ISO viscosity grades
• AGMA gear oil classifications
• SAE crankcase and gear oils
• Saybolt viscosity measurements
  

Keeping a reference chart available can help maintenance teams quickly identify the appropriate lubricant for specific equipment requirements.

While viscosity is one of the most important factors when selecting a lubricant, it is not the only consideration. Operating temperature, load conditions, and industry regulations all influence which lubricant will perform best in a given application.

For example, food processing equipment often requires NSF H1 food-grade lubricants, while heavy industrial gearboxes may require specialized additives for extreme pressure protection.

Keller-Heartt supplies a wide range of lubricants designed for these applications, including:

• Industrial gear oils
• Hydraulic oils
• Compressor lubricants
• Food-grade lubricants
• Metalworking fluids

By selecting the proper viscosity grade and lubricant formulation, maintenance teams can help ensure equipment operates efficiently and reliably.