ISO viscosity grades are used internationally to classify hydraulic oils based on kinematic viscosity at a reference temperature. ISO VG grades provide a standardised framework for specifying viscosity requirements in hydraulic system documentation.
This page explains how ISO VG grades are defined, how they are referenced in hydraulic system specifications, and how individual grades relate to operating conditions.
What ISO VG classification represents
ISO VG grades are defined by ISO 3448 and classify industrial lubricants according to kinematic viscosity measured at 40 °C. Each ISO VG number represents a midpoint viscosity value with defined tolerance limits.
The classification system enables consistent communication of viscosity requirements across equipment manufacturers, lubricant suppliers, and technical documentation.
Use of ISO VG grades in hydraulic specifications
Hydraulic system documentation typically specifies an ISO VG grade or an acceptable viscosity range based on system design, operating temperature, and component sensitivity.
ISO VG classification defines viscosity only and does not describe additive system, base oil type, or performance category.
Common ISO VG grades in hydraulic systems
The most frequently referenced ISO VG grades in hydraulic systems include ISO VG 32, ISO VG 46, and ISO VG 68. These grades are selected based on ambient temperature range, operating temperature, system load, and component clearances.
Lower ISO VG grades are generally associated with lower temperature or higher speed applications, while higher ISO VG grades are typically specified for warmer conditions or higher load environments.
Relationship to other standards
ISO VG classification is often used alongside other standards such as DIN performance classifications and ASTM test method references. Together, these standards define viscosity, test methodology, and minimum performance expectations.
ISO VG grades should be interpreted as one element of the overall hydraulic fluid specification.