Viscosity is the single most important property of any lubricant, and the ISO viscosity grade (ISO VG) is the standard the entire industrial lubrication world uses to talk about it. Get the number right and the oil does its job quietly for years. Get it wrong and you’re looking at excess heat, energy waste, or premature wear that shows up on your maintenance log long before it should.
This guide covers what ISO VG actually measures, how to read a viscosity grade chart, what happens when the grade is wrong, and a simple process for confirming you’re ordering the correct oil, whether that’s for a hydraulic system, a gearbox, or a compressor.
Viscosity is a measure of how easily a fluid flows. A high-viscosity oil is thick and moves slowly; a low-viscosity oil is thin and moves freely. Neither is “better”: the right viscosity is whatever your equipment manufacturer designed the machine around.
In a running machine, viscosity is what separates two moving metal surfaces. That thin film of oil is the only thing standing between smooth operation and metal-on-metal contact. Too thin, and the film breaks down under load. Too thick, and the oil can’t circulate fast enough to get where it’s needed, dragging on components and generating heat as it goes.
ISO VG stands for International Organization for Standardization Viscosity Grade. It’s the classification system used worldwide to grade industrial lubricants by their kinematic viscosity, measured in centistokes (cSt) at 40°C.
The scale runs across 18 standard grades, from ISO VG 2 up to ISO VG 1500. The number itself is the midpoint kinematic viscosity in cSt, with a tolerance of ±10%.
So an ISO VG 46 oil has a target viscosity of 46 cSt at 40°C, and any oil sold under that grade must fall between 41.4 and 50.6 cSt to qualify. That tolerance band is what lets different manufacturers all produce an “ISO VG 46” that behaves the same way in your equipment, regardless of brand.
It’s worth noting this is a viscosity classification, not a quality or performance rating. Two ISO VG 46 oils can carry the same number and still be built for completely different jobs, more on that below.
These are general guidelines only. Always check your equipment manufacturer’s specification before ordering, since the same machine type can call for different grades depending on load, speed and ambient temperature.
Getting viscosity wrong doesn’t usually cause immediate failure. It causes a slow, expensive one.
The oil film can’t hold up under load. That leads to metal-to-metal contact, faster wear on bearings and gears, rising operating temperatures, and a shorter service life for components that are often expensive and slow to replace.
The oil creates drag the system has to work against. That means higher energy consumption, more heat generated by friction, harder cold starts, and in some cases the oil simply can’t get into tight clearances fast enough to lubricate properly at all.
For a business running production equipment, this isn’t an abstract technical point. It shows up directly in energy bills, unplanned downtime, and how often you’re replacing parts that should have lasted years.
The ISO VG number tells you viscosity at a single, fixed temperature: 40°C. It doesn’t tell you what happens to that oil as your equipment heats up or cools down during operation, and in the real world, temperature rarely sits still.
That’s where viscosity index (VI) comes in. VI measures how much an oil’s viscosity changes with temperature. A high-VI oil stays closer to its target thickness across a wide temperature range; a low-VI oil thins out fast as it heats up and thickens fast as it cools.
VI matters most in equipment that sees large temperature swings: outdoor hydraulics through a UK winter and summer, for example, or machinery that runs cold on start-up and hot under load. If that describes your application, the VI figure on the technical data sheet deserves as much attention as the VG number itself.
They’re not directly interchangeable, and the numbers don’t map onto each other in any simple way.
If you’re sourcing oil for a piece of industrial equipment, you want the ISO VG number from the manufacturer’s manual. If you’re sourcing for a vehicle, you want the SAE grade. Mixing the two up when comparing products is a common and avoidable mistake.
A quick guide to finding the correct ISO VG grade for your requirements.
Once you know the grade you need, Velocity Edge lists products by ISO viscosity grade across every major brand, so you can search and compare like-for-like without the back-and-forth of phone calls and quote requests.
If you’re not sure which grade your equipment needs, or you can’t find the exact product you’re used to, email our team with the make and model and we’ll help you find the right match, backed by over 250 years of combined lubrication experience across our technical team.
Below are some answers to some commonly asked questions about ISO VG.
What does ISO VG 46 mean?
ISO VG 46 means the oil has a target kinematic viscosity of 46 centistokes (cSt) when measured at 40°C, with an acceptable tolerance range of 41.4 to 50.6 cSt. It’s one of the most common grades, widely used in hydraulic systems and general industrial machinery.
How many ISO viscosity grades are there?
There are 18 standard ISO VG grades, ranging from ISO VG 2 (very thin) up to ISO VG 1500 (very thick). Each grade is spaced so the midpoint viscosity increases roughly logarithmically up the scale.
Is a higher ISO VG number better?
No. A higher number simply means a thicker oil; it’s not a measure of quality. The correct grade is whatever your equipment manufacturer specifies. Using a higher grade than specified can cause as many problems as using a lower one.
Can I substitute a different ISO VG grade if the one I need is out of stock?
Not without checking with a specialist first. Even a single grade difference can affect lubrication film strength, energy consumption, and component wear. If your exact grade isn’t available, get advice before substituting rather than assuming the nearest number will do.
What’s the difference between ISO VG and SAE viscosity grades?
ISO VG is used for industrial lubricants (hydraulics, gearboxes, compressors) and is measured in cSt at 40°C. SAE grade is used for automotive engine and gear oils and follows a different rating system entirely. The two aren’t directly comparable.
Does ISO VG tell me everything I need to know about an oil?
No. It tells you the thickness of the oil at 40°C, but two oils with the same ISO VG number can have different base oils, additive packages, and intended applications. Always confirm the oil type and any OEM approvals alongside the grade.