Most hydraulic oil decisions are made by copying a grade out of the machine manual. That works until the workshop ambient changes, the relief valve setting is raised, or the reservoir runs ten degrees hotter in summer than the manual ever assumed. The ISO VG number is not the specification. The specification is the viscosity the oil actually has at the pump, and that is a number you have to calculate rather than look up.
What ISO VG actually fixes, and what it does not
ISO 3448 assigns each grade a nominal kinematic viscosity at 40 °C with a permitted range of plus or minus 10 %. The number tells you the band. It tells you nothing about where on the temperature curve the oil will sit once the machine is running.
| ISO VG | Kinematic viscosity at 40 °C (mm²/s) | Where it is normally found |
|---|---|---|
| VG 10 | 9.0 – 11.0 | Cold start-up, high-speed lightly loaded circuits, precision spindles |
| VG 22 | 19.8 – 24.2 | Machine tools, high-speed vane pumps, cool workshops |
| VG 32 | 28.8 – 35.2 | General industrial hydraulics, compressors, temperate plants |
| VG 46 | 41.4 – 50.6 | The default grade for most industrial hydraulic systems |
| VG 68 | 61.2 – 74.8 | Warmer plants, higher system pressure, mobile equipment |
| VG 100 | 90 – 110 | Large presses, mining, high ambient, heavy duty cycles |
The number that decides wear sits at the pump, not at 40 °C
Component makers do not publish a preferred grade, they publish an operating window. For most vane and piston pumps the optimum band is roughly 16 – 36 mm²/s, with an absolute floor near 13 mm²/s at the hottest point of the circuit. Some makers accept 10 mm²/s, but that is a survival limit rather than a design target. At the other end, above roughly 860 mm²/s the oil is too thick for the pump to draw on a cold start, which is why winter grade selection is a separate problem from summer grade selection.
There is a second reason not to work from the tank temperature. Oil leaving a pump, and oil crossing a relief valve, runs about 15 – 25 °C hotter than the reservoir gauge reads. The film temperature at the pump is what you design against, and it is consistently 15 – 25 °C worse than the number the operator can see.
Where each grade falls out of the window
Applying published viscosity-temperature behaviour for a mineral anti-wear hydraulic oil of viscosity index 95 – 100 gives the temperature at which each grade drops through the 13 mm²/s floor:
| ISO VG | at 40 °C | at 60 °C | at 80 °C | Crosses 13 mm²/s at about |
|---|---|---|---|---|
| VG 10 | 10.0 | 5.8 | 3.8 | 32 °C |
| VG 22 | 22.0 | 11.0 | 6.4 | 55 °C |
| VG 32 | 32.0 | 14.9 | 8.3 | 64 °C |
| VG 46 | 46.0 | 20.5 | 11.0 | 74 °C |
| VG 68 | 68.0 | 28.5 | 14.6 | 84 °C |
| VG 100 | 100.0 | 39.9 | 19.6 | 94 °C |
All viscosities in mm²/s. The 40 °C and 100 °C columns are published typical values for mineral HM hydraulic oils with a viscosity index of about 95 – 100; the 60 °C and 80 °C columns and the crossing temperature are calculated from those two anchors with the ASTM D341 viscosity-temperature relation. Confirm the figures against the individual product data sheet before specifying.
The pattern is what matters. Each step up the grade ladder buys roughly 10 °C of headroom before the film falls through the floor. A plant whose film temperature reaches 80 °C cannot be run on VG 22 at all, however convenient that grade is to keep on the shelf, and a plant whose film temperature reaches 60 °C has no margin left on VG 22 either.
What that means for the two light grades
VG 10 and VG 22 are not simply thin versions of VG 32. At 40 °C a VG 10 oil sits at around 10 mm²/s, which is already below the minimum film threshold, and it drops to about 5.8 mm²/s by the time the oil reaches 60 °C. VG 10 is therefore a cold-environment, high-speed, lightly loaded fluid, not a general-purpose choice, and in a circuit that warms up at all it is a wear risk.
VG 22 is the lightest grade that still makes sense in a workshop. It buys roughly 20 °C of headroom over VG 10, which is why it dominates machine tool hydraulics and high-speed vane pump circuits, and it gives way to VG 32 or VG 46 as soon as the plant runs warm or the pressure climbs. Neither light grade should be selected on pump size alone.
The three grades we stock
Our long-life clean-type anti-wear hydraulic oil is supplied in three ISO VG bands, all to GB/T 11118.1 (L-HM, equivalent to ISO 6743-4 HM). Pour point and flash point are the two numbers that reveal which end of the temperature range each grade was formulated for.
| Product | ISO VG band (mm²/s at 40 °C) | Pour point | Flash point (open cup) | Where it fits |
|---|---|---|---|---|
| Long-Life Clean-Type Hydraulic Oil HD HM T10 | VG 10 (9.0 – 11.0) | ≤ -33 °C | 170 °C | Cold start-up, high-speed lightly loaded circuits, low ambient |
| Long-Life Clean-Type Hydraulic Oil HD HM T22 | VG 22 (19.8 – 24.2) | ≤ -33 °C | 200 °C | Cool plants, machine tools, high-speed vane pumps |
| Long-Life Clean-Type Hydraulic Oil HD HM T100 | VG 100 (90 – 110) | ≤ -21 °C | 240 °C | High ambient, high pressure, large presses, mining |
T10 and T22 share a pour point of -33 °C or lower, so both are built for cold start-up, and the difference between them is the working temperature of the circuit rather than the weather. T100 pairs a 240 °C open-cup flash point with a -21 °C pour point, which is the signature of a heavier and more highly refined base stock intended for machines that run hot and loaded. All three carry the same additive package for anti-wear performance, oxidation and ageing stability, anti-corrosion protection with seal compatibility, air release and demulsibility, so a site that needs two grades can standardise on one system without re-qualifying seals or filter elements.
What GB/T 11118.1 actually requires
The GB/T 11118.1-2011 L-HM quality index table is worth reading before any grade is agreed, because it shows how much of the choice the standard leaves open. For the tabulated grades 32 to 100 the requirements are:
| Item | VG 32 | VG 46 | VG 68 | VG 100 |
|---|---|---|---|---|
| Kinematic viscosity at 40 °C (mm²/s) | 28.8 – 35.2 | 41.4 – 50.6 | 61.2 – 74.8 | 90 – 110 |
| Viscosity index, minimum | 95 | 95 | 95 | 95 |
| Flash point, open cup (°C), minimum | 175 | 185 | 195 | 205 |
| Pour point (°C), maximum | -15 | -9 | -9 | -9 |
| Air release value at 50 °C (min), maximum | 6 | 10 | 13 | report |
| Demulsibility at 54 °C (min), maximum | 30 | 30 | 30 | — |
| Copper strip corrosion (100 °C, 3 h), maximum | 1 | 1 | 1 | 1 |
Two things follow from this table. First, the standard is a floor rather than a target: a VG 100 product only has to reach -9 °C pour point and 205 °C flash point to comply, whereas ours is specified at -21 °C and 240 °C. That margin is the difference between an oil that survives a cold plant start and one that does not, and between an oil that resists thermal oxidation at a hot relief valve and one that varnishes it.
Second, note that viscosity index is a minimum of 95 across the family, but the standard does not tell you the viscosity index improver content. A shear-stable VI 95 fluid and a VI 150 fluid thickened with a polymer that shears down in service will both pass on delivery and behave very differently after 2,000 hours. If the application is mobile equipment or a high-pressure circuit, ask for the shear stability result rather than accepting the delivered VI alone.
Cleanliness decides whether the grade you chose survives
Industry studies attribute roughly 75 – 80 % of hydraulic component failures to fluid contamination rather than mechanical fatigue. Cleanliness is reported under ISO 4406 as three codes counting particles larger than 4, 6 and 14 µm per millilitre; the older NAS 1638 scale is still quoted on US-origin equipment. The two count differently and cannot be converted exactly, so the cross-reference below is the working approximation rather than an identity:
| ISO 4406 | Approximate NAS 1638 | Circuit |
|---|---|---|
| 16/14/11 | Class 5 | Servo valve circuits, precision hydraulics |
| 17/15/12 | Class 6 | Proportional valve circuits |
| 18/16/13 | Class 7 | General industrial systems; common new-oil specification in drums |
| 19/17/14 | Class 8 | Gear pumps with directional valves only |
The target is set by the most sensitive component in the circuit, not by the pump. Once a servo or proportional valve is fitted anywhere in the system, the whole circuit inherits the tighter target, because all the oil eventually reaches every component. Retrofitting a proportional valve onto a machine originally specified at 19/17/14 without upgrading filtration at the same time is one of the fastest ways to destroy it.
What to put on the purchase order
- Grade and standard: the ISO VG number together with GB/T 11118.1 L-HM (ISO 6743-4 HM)
- Viscosity index, measured rather than merely claimed, since the standard floor is 95
- Pour point and open-cup flash point, because together they reveal the base stock
- Air release value and demulsibility, the two properties that decide how fast the system responds and how well it sheds water
- Cleanliness as an ISO 4406 code or NAS class, matched to the most sensitive valve in the circuit
Three oil-analysis limits that mean change it
- Viscosity drifting more than 10 % from the new-oil value, which indicates thermal cracking, shear-down of the viscosity index improver, or cross-contamination
- Total acid number rising 0.3 – 0.5 mg KOH/g above the new-oil baseline, which signals that the antioxidant reserve is exhausted
- Water above 0.1 %, after which corrosion, additive depletion and bacterial growth follow quickly
If the reservoir runs hot, fix the cooling
If the reservoir temperature sits above 60 °C in normal operation, the correct first response is heat rejection: clean the heat exchanger, check the flow through it, and review the relief valve settings. Moving up the viscosity ladder to compensate for a cooling problem raises the film temperature further and makes the situation worse rather than better. Grade selection sets the starting point; the cooling circuit decides where the oil ends up.
Our HD HM T10 in ISO VG 10, HD HM T22 in ISO VG 22 and HD HM T100 in ISO VG 100 long-life clean-type anti-wear hydraulic oils are supplied to GB/T 11118.1 L-HM, with viscosity, viscosity index, pour point, flash point, air release, demulsibility and cleanliness reported on every certificate of analysis. Tell us the pump type, the reservoir temperature and the relief valve setting, and we will quote the grade that keeps the film inside the window, or tell you plainly that the system needs a cooler before it needs a different oil. For the base-stock side of high-temperature lubricants, see our guide to polyol ester base stocks.