AW32, AW46 and AW68 are the same type of oil in three thicknesses. The number is the ISO viscosity grade measured at 40°C: AW32 sits in a window of 28.8–35.2 mm²/s, AW46 in 41.4–50.6 mm²/s, and AW68 in 61.2–74.8 mm²/s, per ISO 3448. Nothing else in the designation changes. The anti-wear additive package, the base oil quality and the oxidation resistance are not encoded in the number at all.
Pick by two things: what the equipment manufacturer specifies, and the temperature range the machine actually works in. AW32 for colder ambient conditions and higher-speed pumps, AW46 as the general-purpose industrial default, AW68 for hot environments, heavy loads and slower equipment. If the manual names a grade, that grade wins over any rule of thumb on this page.
One clarification resolves a common ordering error before it happens: AW is not an ISO category. It is a market designation for anti-wear hydraulic oil, widely used in North America. The international system uses two separate standards: ISO 3448 for the viscosity grade, and ISO 6743-4 for the service family. An oil sold as AW 46 is an anti-wear oil at ISO VG 46, which corresponds to the HM family in ISO 6743-4. ISO 46 and AW 46 are not competing specifications, and they are not synonyms either: one names a viscosity, the other names a viscosity plus an additive intent.
What the numbers actually mean
Every ISO viscosity grade is a band, not a single value. ISO 3448 names each grade for the midpoint of its window and allows ±10% either side, measured as kinematic viscosity at 40°C.
| Grade | Midpoint at 40°C | Permitted range | Rough position |
|---|---|---|---|
| AW32 (ISO VG 32) | 32 mm²/s | 28.8 – 35.2 mm²/s | Thinner |
| AW46 (ISO VG 46) | 46 mm²/s | 41.4 – 50.6 mm²/s | The industrial default |
| AW68 (ISO VG 68) | 68 mm²/s | 61.2 – 74.8 mm²/s | Thicker |
The grades step in roughly 1.5× increments, so each one is meaningfully different from the last without wasteful overlap. Two products in the same grade can still differ by about 20% in measured viscosity and both be correctly labelled.
The 40°C reference point matters more than it first appears. It approximates the operating temperature of a lot of industrial equipment, which is why the standard uses it. It also means the grade tells you nothing about how the oil behaves on a cold morning or in a reservoir running at 70°C. That gap is where most selection mistakes live, and it is covered further down.
AW, ISO VG and HM: three labels, one product
Buyers searching for the difference between ISO 46 and AW 46 are asking a reasonable question, because the market uses both terms without explaining either.
- ISO VG 46 is a viscosity grade from ISO 3448. It describes thickness at 40°C and says nothing about additives.
- AW is a market designation meaning anti-wear. It is not defined by ISO. It signals that the oil carries an anti-wear additive package, usually zinc-based, that protects pump surfaces under load.
- HM is the service family from ISO 6743-4 covering refined mineral oils with anti-wear, anti-rust and anti-oxidation properties. Chinese manufacturers use the matching L-HM designation, as on our own high-pressure anti-wear hydraulic oil data sheet.
So an oil described as AW 46 in one catalogue and L-HM 46 in another is, at family level, the same kind of product. The equivalence holds as a category, not as a certification: two HM oils can meet the same family definition and still differ in additive quality, filterability and oxidation life. Treat the family name as a starting filter, then read the technical data sheet.
Choosing by temperature and equipment
The manufacturer’s manual is the first authority. Where it gives a range rather than a single grade, or where a machine has moved to a different climate, these are the working rules.
| Choose | When | Typical equipment |
|---|---|---|
| AW32 | Cold ambient conditions, high-speed pumps, tight clearances, systems where cold-start flow matters most | Machine tools, high-speed vane pumps, cooler indoor plants |
| AW46 | Moderate temperatures, general industrial duty, the default when the manual allows a range | Injection moulding, presses, most factory hydraulics |
| AW68 | Hot ambient or high operating temperature, heavy loads, slower equipment, older systems with worn clearances | Steel mills, outdoor equipment in hot climates, heavy presses |
Two adjustments apply on top of that table. A system running hotter than designed loses viscosity as it heats, so moving up a grade can compensate. Do that only after ruling out the actual cause of the heat, which is a separate diagnosis covered in hydraulic system overheating. A system that cold-starts far below freezing may need something the AW range does not offer at all, which is the limitation the next section deals with.
What the grade does not tell you
Here is the failure mode that catches experienced buyers. Two oils can carry the same AW 46 label, sit inside the same ISO 3448 window, and behave nothing alike once the temperature moves away from 40°C.
These are measured values from two hydraulic oils in current production, both at ISO VG 46:
| Property at ISO VG 46 | Anti-wear grade (L-HE) | Low-temperature grade (L-HV) |
|---|---|---|
| Kinematic viscosity at 40°C | 45.12 mm²/s | 45.47 mm²/s |
| Viscosity index | 105 | 144 |
| Pour point | −21°C | −49°C |
| Flash point (open) | 226°C | 205°C |
At 40°C the two are almost indistinguishable, and the grade label treats them as interchangeable. The viscosity index says otherwise: 105 against 144 means the low-temperature oil holds usable viscosity across a far wider band. The pour points differ by 28°C. In a machine that starts at −25°C, one of these oils flows and the other does not, and no part of the AW 46 designation warns you.
This is why viscosity index and pour point belong on the purchase order alongside the grade. The full comparison of anti-wear against high-viscosity-index formulations is in L-HM vs L-HV hydraulic oil, and the window concept behind the grades is set out in the ISO viscosity grades chart.
What going wrong actually costs
Grade errors rarely announce themselves. They show up as symptoms that get blamed on components.
Too thin for the conditions. The oil film thins under load and pressure, metal-to-metal contact rises, and internal leakage past worn clearances increases. Leakage converts pump energy into heat rather than work, so the system runs hotter, which thins the oil further. Pump wear accelerates and efficiency drops before anything obvious fails.
Too thick for the conditions. Cold-start flow suffers, the pump can cavitate as it struggles to draw oil, and fluid friction generates heat of its own. Response goes sluggish, and cavitation damage to the pump inlet is cumulative and permanent.
Both directions end in the same place: a hotter system and a shorter pump life. That is why the symptom alone cannot tell you which way you erred. The temperature range and the manual can.
Beyond the AW range
The AW family covers most industrial hydraulics, but three situations sit outside it.
Genuine cold climates. Below roughly −25°C ambient, a standard anti-wear oil is too stiff to pump at start-up regardless of grade. A high-viscosity-index formulation such as L-HV low-temperature hydraulic oil is designed for this, and L-HS extreme-low-temperature oil extends the pour point further.
Fire risk. Near furnaces, die-casting cells or molten metal, mineral oil is a hazard whatever its grade. Fire-resistant and less flammable fluids form a separate family under ISO 12922, with their own trade-offs in seal compatibility and pump limits.
High cleanliness requirements. Servo valves and proportional systems fail on contamination before they fail on viscosity. A high-cleanliness grade such as L-HE is specified by filterability and cleanliness class, not by the AW number.
What to put on the purchase order
The grade alone underspecifies the product. A complete request includes:
- ISO viscosity grade: 32, 46 or 68, taken from the equipment manual.
- Service family: HM for standard anti-wear, HV where the temperature range is wide, or the equipment maker’s own specification if it names one.
- Viscosity index and pour point: the two values that decide cold-start behaviour, and the ones the grade hides.
- Any OEM approval the system requires: pump makers publish their own fluid requirements, and these override general category rules.
- The technical data sheet for the exact grade and batch. A supplier who cannot produce one for the grade being quoted is a supplier worth a second look. The wider checklist is in our lubricants sourcing guide.
FAQ
Is AW 46 the same as ISO 46? Not quite. ISO VG 46 names the viscosity grade only. AW 46 means an anti-wear oil at that grade. Every AW 46 is an ISO VG 46, but an ISO VG 46 oil is not necessarily an anti-wear formulation.
Can I use AW46 instead of AW32? Only if the equipment manual permits the range. AW46 is thicker, so cold-start flow suffers and fluid friction rises. In a machine specified for AW32 in a cool plant, the substitution usually costs efficiency rather than causing immediate damage, but it is still a deviation from specification.
Can I mix AW32 and AW46? Mixing two anti-wear oils of the same family and base type is generally tolerated in an emergency, and the result is an intermediate viscosity that matches neither grade. It is a way to finish a shift, not a way to run a system. Drain and refill with the specified grade.
What is the difference between AW32 and AW46 in one sentence? Thickness at 40°C, and nothing else in the designation.
Which grade is most common? AW46 (ISO VG 46) is the general-purpose industrial default in temperate conditions, which is why it is the most widely stocked grade.
Does a higher AW number mean better protection? No. The number is a viscosity grade, not a quality or protection rating. Anti-wear performance comes from the additive package and is documented on the data sheet, not encoded in the grade.
Sources
- ISO 3448 — industrial liquid lubricants, ISO viscosity classification. Defines the VG 32, 46 and 68 windows quoted above.
- ISO 6743-4 — classification of hydraulic fluids by service family, including the HM and HV families.
- Viscosity, viscosity index, pour point and flash point figures for the ISO VG 46 comparison: Zhongtian Petrochemical technical data sheets for L-HE and L-HV, current production.
Zhongtian Petrochemical has manufactured hydraulic oils since 1998. Our quality, environmental and occupational health management systems are certified to ISO 9001:2015, ISO 14001:2015 and ISO 45001:2018 under registration numbers 62825Q9372R0S, 62825E9373R0S and 62825S9374R0S, valid to April 2028 and verifiable through the issuing body. Our anti-wear range is produced in ISO VG 32, 46, 68, 100 and 150.
If you are matching a grade to a specific pump, send us the equipment make, the specification in the manual and the site’s temperature range, and our technical team will specify against them. Contact Zhongtian Petrochemical.