Understanding Hydraulic Oil Viscosity
Hydraulic oil viscosity is typically rated using the ISO VG (Viscosity Grade) system, which specifies kinematic viscosity at 40°C in centistokes (cSt). AW32, AW46, and AW68 correspond to ISO VG 32, 46, and 68 respectively — the numbers represent the nominal viscosity in cSt at 40°C.
The "AW" designation stands for Anti-Wear — meaning the oil contains additives specifically designed to protect metal surfaces in high-pressure pump and valve contacts. AW hydraulic oils are the most widely used type in industrial and mobile equipment, distinct from plain mineral oils or fire-resistant hydraulic fluids.
Key principle: Viscosity controls whether the oil creates an adequate film between moving parts. If the oil is too thin for the operating pressure and temperature, metal-to-metal contact occurs. If it is too thick, the pump works harder than necessary, generating excess heat and wasting energy.
The Three Main Grades Compared
| Grade | Viscosity at 40°C | Best Operating Temp | Typical Application |
|---|---|---|---|
| AW32 | ~32 cSt | 20°C – 45°C | Machine tools, indoor industrial, cold climate mobile |
| AW46 | ~46 cSt | 35°C – 55°C | Most industrial hydraulic systems, presses, injection moulding |
| AW68 | ~68 cSt | 50°C – 70°C | High-pressure systems, outdoor mobile, hot ambient conditions |
AW32 — Lighter Duty and Cold Conditions
With the lowest viscosity of the three, AW32 flows readily even at lower temperatures. It is well-suited for indoor industrial machinery operating in air-conditioned or controlled environments, precision machine tools such as lathes and grinding machines, and mobile hydraulic systems operating in cold climates where the oil temperature rarely exceeds 45°C. Because it flows easily, AW32 delivers responsive control and low startup resistance in cold conditions. However, it provides less film protection under high loads or at elevated temperatures — if the system runs hot, AW32 will thin out excessively and leave metal surfaces under-protected.
AW46 — The General-Purpose Standard
AW46 is the most widely used hydraulic oil grade globally, and for good reason — it balances flow characteristics, film strength, and temperature range across a broad set of operating conditions. The majority of industrial hydraulic power units, presses, injection moulding machines, and fork lifts are factory-filled with AW46. If you are operating industrial equipment in India's climate — where ambient temperatures in most states range from 20°C to 40°C — AW46 is typically the correct grade for indoor machinery. Haruto NexaGuard hydraulic oil is available in AW 46, alongside AW 32 and AW 68.
AW68 — Heavy Duty and Hot Environments
AW68's higher viscosity provides a thicker oil film at elevated temperatures, making it the right choice when the hydraulic oil routinely reaches 50°C or above. This includes outdoor mobile equipment operating in hot climates — excavators, agricultural machinery, crane trucks, and road-building equipment running through Indian summers — as well as high-pressure industrial systems where the oil absorbs significant heat from pump and valve work. The additional film thickness of AW68 also helps protect older systems where pump clearances have widened due to wear, as the thicker oil is more resistant to internal leakage across the pump.
ISO VG Bands: What the Numbers Actually Allow
Each ISO VG number is the mid-point of a band, not an exact value. The ISO viscosity grade system allows 10% either side of the mid-point, measured at 40 °C:
| Grade | Viscosity at 40 °C | In practice |
|---|---|---|
| ISO VG 32 (AW32) | 28.8–35.2 cSt | Easy cold starts and quick response; least film margin when hot |
| ISO VG 46 (AW46) | 41.4–50.6 cSt | The middle ground most industrial machines are built around |
| ISO VG 68 (AW68) | 61.2–74.8 cSt | Thicker film when hot; slower to flow when cold |
The bands do not overlap: if a data sheet figure at 40 °C falls outside the band, the oil is not the grade on the label. And 40 °C is only a reference point. What the pump sees is the viscosity at the oil's working temperature, which in an Indian plant in May is usually well above 40 °C. How fast an oil thins as it heats is described by its viscosity index (VI): mineral anti-wear oils typically sit around a VI of 100; high-VI oils thin much less. Our guide to hydraulic oil for JCB and excavators covers VI and cold starts on mobile machines.
How System Design Affects Grade Selection
Operating pressure matters alongside temperature. Systems running at very high pressures (above 250–300 bar) generate more heat internally and tend to thin the oil faster — this is an argument for choosing a higher-viscosity grade. Conversely, systems with servo valves and proportional controls often specify lower-viscosity oils for faster response and lower pressure drop across the spool.
Pump type also matters. Gear pumps are generally more tolerant of viscosity variation. Axial piston pumps — particularly variable-displacement units common in industrial machinery — are more sensitive: both excessively thin and excessively thick oil can affect efficiency and cause premature internal wear.
Reservoir size and cooling determine how hot the oil actually runs in service. A large, well-cooled reservoir keeps oil temperatures lower, potentially allowing a lighter grade. A small, poorly cooled system in a hot environment will run the oil significantly hotter than ambient, pushing the selection toward a heavier grade.
Common error — mixing grades: Topping up with a different grade when the correct one is unavailable is a frequent cause of hydraulic problems. Mixing AW32 and AW68, for example, produces an oil that meets neither viscosity specification. Always use the grade specified for the system, and if switching grades during a maintenance interval, drain and flush the system first.
The Pump Maker's Viscosity Window
The most reliable way to choose between AW32, AW46 and AW68 is not a rule of thumb but the pump data sheet. Most pump makers publish three viscosity limits:
- Minimum viscosity — the thinnest the oil may become at the highest operating temperature, usually only briefly. Below it the pump's internal film breaks down and leakage rises.
- Optimum range — where the pump is most efficient and wears least; the oil should sit here for most of the shift.
- Maximum start-up viscosity — the thickest oil the pump can draw on a cold start without cavitating.
The limits differ by pump type and maker, so the data sheet beats general advice. Take the oil's viscosity at 40 °C and 100 °C from its product data sheet, ask the supplier for a viscosity–temperature chart, and read off the temperatures at which the oil crosses each limit. The right grade is the one whose optimum range lines up with your measured running temperature. Where a machine has more than one pump, the most sensitive pump sets the grade.
Measure the Real Oil Temperature in Your Plant
The grade table above is keyed to oil temperature, not room temperature, yet few plants measure it. A week of readings settles most grade arguments:
- Read the reservoir thermometer, or use an infrared thermometer on the return line near the tank.
- Take readings at start-up, mid-shift and end of the longest shift in the hottest weeks — across Gujarat, Rajasthan and central India, typically April to June. Note the shed temperature too; tin-roofed sheds often run far hotter than outdoors.
- In north India, also take a reading before the first shift on a winter morning to check cold-start conditions.
- Compare the stable running temperature with the grade table and the pump's viscosity window.
If the oil runs hotter than the maker expects, find the cause before moving up a grade: a scaled heat exchanger, dust-choked cooler fins, a worn pump, a relief valve set too low or a low level. A heavier grade masks these faults; it does not fix them.
Signs You Have the Wrong Grade
If the oil is too thin: the system runs hotter than normal, pump noise increases (cavitation or metallic chatter), and wear rates on pump and valve components accelerate. You may also notice increased internal leakage — cylinders creeping under load, or the system losing pressure at rest.
If the oil is too thick: the system is sluggish on cold startup, the hydraulic pump struggles to prime, filter pressure differentials are higher than normal, and energy consumption increases. In cold climates, an overly viscous oil can make the system nearly inoperable until it reaches operating temperature.
Is It the Grade or Something Else?
Many complaints blamed on the oil grade start somewhere else. Before changing grades, work through the likely causes:
| Symptom | Grade-related cause | Other common causes |
|---|---|---|
| Oil running hot | Too thin, so internal leakage turns into heat | Dirty or scaled cooler, low level, relief valve dumping, worn pump |
| Pump whine at start-up | Too thick for the cold-start temperature | Blocked suction strainer, air leak on the suction side, low level |
| Cylinders slow or creeping | Too thin at working temperature | Worn seals, internal valve leakage, worn pump |
| Sticking or erratic valves | Rarely the grade | Varnish from oxidised oil, fine dirt silting the spool |
| Foam in the tank | Incompatible oils mixed at top-up | Air drawn in at the suction line, return line ending above the oil level |
| Filters blocking early | Too thick in cold conditions | Dirty new oil, oxidation products, a failing component |
Additional Specifications to Check
Viscosity grade is the starting point, but confirm the full specification before ordering. Key additional requirements include the antiwear performance standard (DIN 51524 HLP is the industrial benchmark in most markets), oxidation stability (longer drain intervals demand better oxidation resistance), zinc-free or ashless formulation (some OEMs, particularly for sensitive servo systems, specify zinc-free AW hydraulic oils), and foam resistance and air release characteristics for systems with small reservoirs or high recirculation rates.
HM, HV, HL and Zinc-Free: Choosing the Oil Family
The viscosity grade tells you how thick the oil is; the family tells you what it is built to do. ISO 11158 and DIN 51524 use different labels for the same families:
| ISO 11158 | DIN 51524 | What it is | Typical use |
|---|---|---|---|
| HL | Part 1 (HL) | Rust- and oxidation-inhibited oil without anti-wear additive | Low-pressure circulating systems, only where the maker allows it |
| HM | Part 2 (HLP) | Anti-wear oil — the AW oils in this guide | Most industrial pumps, presses, moulding machines and power packs |
| HV | Part 3 (HVLP) | Anti-wear oil with a high viscosity index | Wide temperature swings, outdoor and mobile equipment, cold starts |
HV oils usually get their high VI from polymer additives, which heavy shearing can break down over time; ask for shear-stability data when an HV oil is specified. Zinc-free (ashless) oils are not a separate class: they are HM or HV oils that use zinc-free anti-wear chemistry. Use them where the pump or servo-valve maker calls for it, and do not mix zinc-based and zinc-free oils without the supplier confirming compatibility.
Haruto NexaGuard hydraulic oils are anti-wear oils offered in AW 32, AW 46 and AW 68 in packs from 1 L to 210 L. If your manual calls for an HV or zinc-free oil, follow the manual and share the specification with your supplier.
Grade Selection by Machine Type
Typical starting points for Indian plants and sites. The equipment manual decides; use this to sense-check a recommendation, not override it.
| Machine | Typical grade | Notes |
|---|---|---|
| Plastic injection moulding machines | AW46 | AW68 only if the maker permits it for hot sheds; keep the cooler clean |
| Hydraulic presses and power packs | AW46 | AW68 for large high-pressure presses that run hot; AW32 for small units in cool rooms |
| CNC and conventional machine tools | AW32 | Slideways and spindles often need their own oils — do not substitute |
| Servo and proportional systems | As specified, often AW32 or AW46 | Viscosity-sensitive; may need zinc-free oil and fine filtration |
| Excavators, backhoes and cranes | AW68 in most Indian conditions | See our excavator guide |
| Tractor hydraulics | Often a shared transmission fluid | See what UTTO oil is |
Changing Grade or Supplier: A Clean Changeover
When you change grade or supplier, do it properly. Old oil left in lines and coolers dilutes the new fill, and mixed additive chemistries can form deposits or foam.
- Confirm the new oil meets the machine's specification and check compatibility with the oil in service.
- Run to working temperature so dirt is in suspension, then shut down, lower all loads and release pressure, including accumulators.
- Drain the reservoir fully, plus low points, cylinders and coolers where practical. Clean the tank with lint-free cloths — never cotton waste, which sheds fibres.
- Fit new filter elements and clean the suction strainer and breather.
- Refill through a filter, bleed air and cycle every function without load. If the old oil was badly degraded or of a different chemistry, flush with a charge of the new oil and drain before the final fill.
- Change the filters again after the initial running period and take an oil sample as your new baseline.
Condition Monitoring: Keep the Grade You Chose
Choosing the grade is half the job; the other half is knowing it is still in grade months later. A basic laboratory analysis checks viscosity at 40 °C (many plants investigate a change of about 10% from the new-oil value), water, acid number as a measure of oxidation, an ISO 4406 particle count, and wear metals and additive elements. Sample from the same point each time, with the system at working temperature — ideally a valve on the return line before the filter — and send a new-oil sample from the drum as a baseline. Trends matter more than single results. For drum handling and storage, see our guide to buying hydraulic oil 68 in bulk.