Viscosity Index vs Viscosity: Read a Hydraulic Oil Specification
Viscosity is temperature-specific; VI describes temperature dependence. Read hydraulic-oil data without treating either as equipment approval.
Viscosity tells you a fluid property at a stated test condition. Viscosity index, or VI, describes how kinematic viscosity changes with temperature. A higher VI is not a higher ISO VG grade, and neither number alone tells a buyer that an oil is suitable for a particular hydraulic system.
The useful comparison starts with three separate questions: what was measured, at which temperature, and what does the equipment permit? Keeping those questions separate prevents a high-VI description from becoming an unsupported cold-start or substitution claim.
Separate the measurement, the grade and the index
Kinematic viscosity is reported in mm²/s, also called cSt; 1 cSt equals 1 mm²/s. The temperature is part of the result. A value at 40°C and a value at 100°C are not two interchangeable descriptions of the same test condition. ASTM D445 identifies the kinematic-viscosity measurement, while ExxonMobil's viscosity technical note distinguishes viscosity from its temperature dependence. Its aviation grades are not hydraulic-oil instructions.
| Item on a specification | What it describes | What it cannot establish alone |
|---|---|---|
| Kinematic viscosity with a test temperature | A measured viscosity, in mm²/s or cSt, at that condition | The complete temperature curve or equipment acceptance |
| ISO VG | An industrial viscosity class referenced to 40°C | Additive system, cold-start permission or OEM approval |
| Viscosity index | A calculated index based on kinematic viscosity at 40°C and 100°C | A viscosity in cSt, a percentage improvement or a universal operating range |
The existing hydraulic-oil grade guide deals with choosing between grades. Here, the distinction is how to read different properties on one offered grade. Do not compare one supplier's viscosity at 40°C with another supplier's result at 100°C as if a larger number identified the thicker offer on a common basis.
Compare the temperature behavior on a common basis
ASTM D2270 calculates VI from the 40°C and 100°C kinematic viscosities; a higher index signifies less viscosity decrease as temperature rises within that framework. VI is an index, not the percentage of viscosity retained. Dividing the two measured viscosities does not reproduce the standard's VI calculation.
For a meaningful shortlist, first hold the required grade and fluid category constant. Read the actual viscosity results, their temperatures and methods beside the stated VI. Two oils in one VG class need not have identical measured viscosity or temperature behavior. An index comparison also cannot replace the actual fluid-viscosity evidence at the machine's service conditions.
Treat an apparent mismatch as a document question before treating it as a product failure. Confirm that the two viscosity inputs and the quoted VI describe the same named grade and data basis. A family brochure, a typical-value table and a particular lot COA may answer different questions. They are not automatically contradictory, but mixing them into one synthetic data row creates a comparison that no supplier actually reported.
Keep cold start and hot operation as separate checks
The input temperatures for VI do not make it a cold-start test. Do not extend a straight line from two warm-temperature values to approve a subzero start. A low pour point is also not a pump-specific starting instruction. Define the lowest fluid temperature at startup separately from the stabilized operating-fluid temperature; ambient temperature alone does not establish either condition.
An actual pump document shows why this matters. Bosch Rexroth's A10VO Series 32 data sheet, page 4 separates operating viscosity, short-term limits and cold-start conditions, and distinguishes reservoir from case-drain temperature. Those conditions belong to that pump series, not every hydraulic system. Use the current instructions for the actual machine rather than copying that example's limits.
Turn the comparison into a specific enquiry
The MOS L-HV hydraulic-oil family lists ISO VG 32, 46 and 68 and describes high viscosity index. Its current native product record does not give a numerical VI with a complete 40°C/100°C pair. This guide therefore assigns no numerical VI, temperature limit or equipment approval to an offered MOS grade.
Send the exact equipment and pump identification, required fluid specification, proposed grade, startup-fluid temperature, service-fluid temperature and present oil identity. Ask for the offered grade's current TDS/SDS, method-identified viscosity data and the evidence required for that equipment. State whether the purchasing requirement is a specification limit or whether a reported value is merely typical; agree the lot acceptance basis separately. Include sample and order quantities, packaging and destination through the existing product enquiry.
The purchasing decision is whether an identified, documented grade meets the system's requirements across its relevant conditions. “Higher VI” helps describe one property; it is not the final acceptance decision.