Explainer · № 005 — TECHNICAL DESK · LUBRICANTS
What a Viscosity Grade Really Tells You.
A number on a drum is not a complete description of the oil inside. Start with the temperature, then read the range.
WORDS: CRUDE AWAKENING EDITORIAL · ARTWORK: GENERATED EDITORIAL ILLUSTRATION
There is an important difference between a measurement and a name. ISO VG 46 looks like a measurement: forty-six centistokes. But on a lubricant label it names a class. Two oils can belong to that class without having exactly the same measured viscosity.
ISO 3448 classifies industrial liquid lubricants by kinematic viscosity at 40°C. Kinematic viscosity is expressed in square millimetres per second, written mm²/s; one mm²/s is one centistoke, or cSt. The grade number is the nominal midpoint, not a promise that every sample will hit that number exactly.
For ISO VG 46, the permitted range at 40°C is 41.4–50.6 cSt. For ISO VG 68, it is 61.2–74.8 cSt. Each range extends ten per cent either side of its nominal midpoint. An oil measuring 45 cSt at 40°C can therefore fall within VG 46; one measuring 70 cSt can fall within VG 68.
A viscosity grade is a classification at one reference temperature. It is not a complete account of how an oil behaves.
The reference temperature matters. Oil viscosity changes with temperature, and the grade alone does not give you a temperature–viscosity curve. You cannot reliably turn VG 46 into a viscosity at 80°C with a universal formula. That requires additional product data. Nor do the designations imply that two curves cross at 40°C: these two ranges are separate there.
The classification also does not establish whether an oil has the right additive system, material compatibility or equipment approval for a particular application. Two products with the same viscosity grade are not necessarily interchangeable. The label answers one question while leaving others open.
For a purchasing decision, put the equipment maker's requirements beside the product data sheet. Check the grade, the stated test temperature, the relevant performance specifications and any required approvals. Where suitability is uncertain, ask the equipment maker or a qualified lubrication specialist. A good technical document is valuable precisely because it tells you more than the largest number on the drum.