Short answer
An ISO VG number is the midpoint kinematic viscosity of an industrial oil in centistokes (mm²/s) at 40 °C, as classified by ISO 3448. An ISO VG 46 oil must measure between 41.4 and 50.6 cSt at that temperature. The grade tells you thickness only; it says nothing about additives, oxidation life or whether the oil suits a particular pump, compressor or gearbox.
How the ISO 3448 system is built
ISO 3448 sets out twenty viscosity grades for industrial liquid lubricants, running from ISO VG 2 up to ISO VG 3200: 2, 3, 5, 7, 10, 15, 22, 32, 46, 68, 100, 150, 220, 320, 460, 680, 1000, 1500, 2200 and 3200. Each number is the midpoint kinematic viscosity at 40 °C, and each step is roughly half again as thick as the one below it.
Every grade allows a tolerance of plus or minus 10% around its midpoint. ISO VG 32 covers 28.8 to 35.2 cSt, ISO VG 46 covers 41.4 to 50.6 cSt and ISO VG 68 covers 61.2 to 74.8 cSt. Because the bands do not overlap, a measured viscosity at 40 °C places an oil in exactly one grade or between grades.
Why 40 °C, and what it leaves out
The 40 °C reference is a convenient laboratory temperature close to the running temperature of many hydraulic and circulating systems. It does not tell you how fast the oil thins as it heats or thickens as it cools. Two ISO VG 46 oils can behave very differently at 0 °C or 80 °C if their viscosity index differs, which is why multi-viscosity (high-VI) hydraulic oils exist.
Data sheets usually list viscosity at both 40 °C and 100 °C, measured by ASTM D445 or ISO 3104, along with the viscosity index. Those extra numbers are what let you estimate viscosity at your real operating temperature.
ISO VG compared with SAE grades
SAE J300 engine oil grades and SAE J306 gear oil grades use different test temperatures and different rules, so there is no exact conversion. Published comparison charts are approximate at best. For example, an SAE 10W engine oil and an ISO VG 32 hydraulic oil can sit in a similar viscosity range at 40 °C, but the additive chemistry is completely different.
If a machine’s manual calls for an ISO VG grade, choose an industrial oil of that grade and type rather than substituting an engine or gear oil on the basis of a conversion chart.
Where you will see ISO VG numbers
ISO VG grades appear on hydraulic oils, compressor oils, turbine and circulating oils, way oils, industrial gear oils and many bearing oils. They are often combined with a performance category, as in ISO HM 46 under ISO 6743-4 or HLP 46 under DIN 51524, where the letters describe the oil type and the number is the ISO VG grade.
Frequently asked questions
Does a higher ISO VG number mean better protection?
No. A higher number only means a thicker oil at 40 °C. The right grade is the one the equipment maker specifies for your operating temperatures; oil that is too thick can starve pump inlets and slow cold starts.
Is ISO VG 46 the same as SAE 20?
There is no exact equivalence. The two systems use different temperatures and limits, so follow the grade named in the equipment manual rather than a conversion chart.
Can I mix two ISO VG grades to get one in between?
Blending changes viscosity, but it can also dilute additive packages and make the oil’s identity unclear. It is better to buy the grade the equipment calls for.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
