Oil Viscosity Grades: ISO VG Explained

ISO VG (ISO Viscosity Grade) is a number that tells you a lubricant's kinematic viscosity at 40 degrees C, nothing more and nothing less. Get the grade wrong on a gearbox or bearing and you are not making a paperwork error, you are choosing how thick the film is between two pieces of steel that are never supposed to touch.

What an ISO VG number actually means

The ISO viscosity classification (originally ISO 3448, with kinematic viscosity determined per ISO 3104 test methodology and mirrored in ASTM D2422) assigns each industrial lubricant to a grade named after its nominal kinematic viscosity in mm2/s (centistokes, cSt) at 40 degrees C. ISO VG 46 means the oil's viscosity midpoint at 40 C is 46 cSt. That is the whole naming logic.

Each grade allows a tolerance band of plus or minus 10 percent around that midpoint. So ISO VG 46 covers 41.4 to 50.6 cSt, and ISO VG 68 covers 61.2 to 74.8 cSt. The system runs 20 grades from ISO VG 2 up to ISO VG 3200, each roughly 50 percent higher than the one before it. Forty degrees C was picked as the reference point because it sits in the normal operating range of most industrial machines, away from cold-start and high-temperature extremes that would distort a comparison.

This is why an oil drum label reading "ISO VG 32" tells a technician exactly what to expect from a viscometer test, and why swapping in an unlabeled or mixed-source oil without checking the grade is a real risk, not a formality.

How ISO VG relates to SAE grades

SAE grades and ISO VG grades measure related things but are not interchangeable definitions. SAE J300 (engine oils) defines multigrade oils by cold-cranking viscosity and pumpability at low temperature (the "W" number) plus kinematic and high-temperature high-shear viscosity at 100 C, not by a 40 C midpoint. ISO VG is purely a 40 C kinematic viscosity classification for industrial fluids: hydraulic, gear, compressor, and circulating oils.

The two systems only line up as an approximate cross-reference, useful for a ballpark comparison, never as a substitution rule:

• ISO VG 22 is roughly comparable to SAE 5W at 40 C
• ISO VG 32 is roughly comparable to SAE 10W at 40 C
• ISO VG 46 is roughly comparable to SAE 15W at 40 C
• ISO VG 68 is roughly comparable to SAE 20W at 40 C
• ISO VG 100 is roughly comparable to SAE 30 at 40 C

Do not read that table as permission to pour SAE 15W-40 engine oil into a system speced for ISO VG 46 hydraulic fluid. The base stocks, additive packages (detergents, dispersants, anti-wear chemistry), and seal compatibility differ even when the 40 C number happens to match. Always follow the OEM's stated ISO VG or SAE grade for the specific machine, not a cross-reference chart.

Viscosity index: how the oil behaves across temperature

ISO VG only tells you viscosity at one temperature. Viscosity Index (VI), calculated per ASTM D2270 (equivalent to ISO 2909) from kinematic viscosity readings at both 40 C and 100 C, tells you how much that viscosity changes as temperature swings. A higher VI means the oil thins out less as it heats up and thickens less as it cools down, which is what you want for equipment that runs across a wide temperature range or starts cold.

The VI scale was set in 1929 by Dean and Davis using two reference crude oil series: Pennsylvania paraffinic crude, which showed the least viscosity change with temperature, was assigned VI 100; Texas Gulf Coast naphthenic crude, which changed the most, was assigned VI 0. Modern synthetic base oils and VI-improved formulations routinely exceed 100, sometimes well past 150, which is why the standard's Procedure B extends the scale beyond the original two reference points.

• Low VI (under about 40): viscosity swings sharply with temperature, generally acceptable only for stable-temperature indoor equipment
• Standard mineral oils: typically VI 90 to 100
• High-VI synthetics and VI-improved multigrades: 120 and above, better for wide ambient swings and mixed-speed cold starts

Why the grade matters for the bearing film

A rolling-element bearing does not run on metal-to-metal contact when it is healthy. It runs on a microscopically thin elastohydrodynamic oil film, often on the order of a micron, that fully separates the rolling elements from the raceway. ISO 281 formalizes this with the viscosity ratio, kappa: the actual operating viscosity of the oil divided by the minimum viscosity that bearing needs at its speed and size. A kappa in roughly the 1 to 4 range is the target zone, with bearing fatigue life improving as kappa climbs toward 4 and dropping off sharply below 1.

Get the grade wrong in either direction and the failure mode changes:

• Too thin (kappa below 1): the film collapses under load, asperities on the rolling elements and raceway make contact, and you get adhesive wear, micro-pitting, and accelerated fatigue. This is a common root cause behind several classic bearing failure modes and symptoms.
• Too thick: churning losses rise, operating temperature climbs, seals work harder, and startup torque increases, especially in cold conditions.

Because oil viscosity itself is temperature-dependent, the practical target grade shifts with ambient and operating temperature, speed, and load, which is exactly why OEM lubrication charts specify an ISO VG grade rather than leaving it to guesswork. If you're troubleshooting a bearing that has already failed, cross-check the lubricant that was in service against the OEM's grade before assuming the root cause was L10 bearing life exhaustion from normal fatigue.

Reading condition signals instead of guessing

Wrong-grade lubrication rarely announces itself immediately. It shows up first as a slow rise in running temperature, then as vibration signature changes as film thickness degrades, long before a bearing seizes. Combining oil analysis with thermography and vibration analysis catches the drift while it is still a lubrication adjustment, not a bearing replacement.

This is also where a maintenance strategy decision matters. Scheduling oil sampling and viscosity checks on a fixed calendar is preventive maintenance; catching an actual viscosity or contamination drift as it happens and routing a work order automatically is closed-loop condition monitoring. Fabrico reads machine condition and OEE signals directly from the line, computer vision catches drift and anomalies that vibration or temperature sensors alone can miss, and a detected loss auto-routes a work order instead of waiting for the next scheduled inspection. It's built and hosted in the EU with EU data residency, and runs under ISO 27001, ISO 20000-1, and ISO 9001. Book a Fabrico demo to see it on your own line.

Frequently Asked Questions

What does ISO VG 46 mean in practical terms?

It means the oil's kinematic viscosity at 40 degrees C falls between 41.4 and 50.6 cSt, with 46 cSt as the nominal midpoint. It says nothing on its own about the oil's viscosity index, additive package, or performance at other temperatures.

Can I substitute ISO VG 68 for ISO VG 46 if that's all I have on hand?

Not as a routine practice. A thicker-than-specified oil raises churning losses and startup drag; a thinner one risks film collapse under load. Use the OEM-specified grade, and treat a substitution as a temporary measure to flag and correct, not a standing fix.

Is a higher viscosity index always better?

Higher VI means more stable viscosity across a temperature range, which generally helps equipment exposed to cold starts or wide ambient swings. But VI is one property among several (base oil type, additive package, oxidative stability) and does not replace matching the correct ISO VG grade for the application.

Why do standards use 40 C and not operating temperature for the ISO VG rating?

Forty degrees C was chosen as a common, repeatable reference point that sits within typical industrial operating ranges without the distortion that cold-start or high-temperature extremes would introduce, so oils from different suppliers can be compared on equal terms.