ISO VG 46 can sometimes replace ISO VG 68 hydraulic oil, but only under specific conditions. Substitution depends on operating oil temperature, system design, load, and manufacturer permission.
Using ISO VG 46 as a replacement for ISO VG 68 without proper evaluation can result in increased wear, internal leakage, and reduced component life.
This page explains when ISO VG 46 may be acceptable, when it is not, and how to assess substitution risk in industrial and mobile hydraulic systems.
What the viscosity difference actually means
ISO VG viscosity grades define oil thickness measured at 40 degrees Celsius.
- ISO VG 46 is thinner than ISO VG 68 at the same temperature
- As temperature increases, viscosity decreases for both grades
This means substitution decisions must be based on operating oil temperature, not nominal viscosity grade alone.
When ISO VG 46 may be acceptable
ISO VG 46 may be suitable as a replacement for ISO VG 68 where:
- Operating oil temperature is consistently lower than originally assumed
- The manufacturer permits multiple viscosity grades
- System load and pressure are moderate
- Internal leakage is not already a concern
In these cases, ISO VG 46 may provide adequate lubrication while improving efficiency.
When ISO VG 46 should not be used
ISO VG 46 should not replace ISO VG 68 where:
- Operating oil temperatures are high
- Systems operate under heavy load or high pressure
- The manufacturer specifies ISO VG 68 only
- Leakage or wear is already present
In these conditions, lower viscosity increases wear and shortens component life.
Temperature is the controlling factor
Viscosity at operating temperature determines lubrication performance.
If viscosity becomes too low at temperature, oil film strength and internal sealing are reduced, as explained in what happens if hydraulic oil viscosity is too low .
Temperature must be measured under normal duty, not inferred from ambient conditions.
System design and pump type matter
Different pump types have preferred viscosity ranges.
- Gear pumps may tolerate a wider range
- Vane and piston pumps are more sensitive to low viscosity
Substitution should never ignore component specific requirements.
Is ISO VG 46 a safer option in cold conditions
ISO VG 46 may improve cold start flow compared to ISO VG 68.
However, this does not justify permanent substitution unless operating temperature remains within acceptable viscosity limits during normal operation.
Where temperature variation is the concern, a high viscosity index hydraulic oil may be a better solution.
Frequently asked questions
Can ISO VG 46 replace ISO VG 68 hydraulic oil
Sometimes. Only if operating temperature, load, and manufacturer specifications permit the lower viscosity.
Is ISO VG 46 better than ISO VG 68
No. Neither is universally better. The correct grade depends on operating conditions.
What is the risk of using ISO VG 46 instead of ISO VG 68
Potential risks include increased wear, leakage, and reduced pump life if viscosity becomes too low at temperature.
Related reference pages
For viscosity fundamentals, refer to what happens if hydraulic oil viscosity is too high and what happens if hydraulic oil viscosity is too low . For viscosity grade comparison, refer to which is better hydraulic oil ISO VG 32 or ISO VG 46 .