Choosing a hydraulic oil: ISO VG, HLP, HVLP and zinc-free

Most hydraulic oil is selected by looking at the label on the drum that was there before. That works until something changes — a new pump, a colder building, a system that now runs hotter than it used to — and then it quietly stops working. The selection has three parts, and each one is decided by something physical.

What the letters mean

The DIN 51524 classification is the one that matters in practice, and the letters are cumulative rather than alternative.

Reading upward: an HVLP is an HLP with a flatter viscosity-temperature curve. It does not replace the anti-wear performance, it adds to it. Specifying HVLP where HLP would do is not wrong, it is simply paying for a property the system does not use.

The viscosity grade belongs to the pump

ISO 3448 defines a viscosity grade by its kinematic viscosity at 40 °C: an ISO VG 46 oil measures 46 mm²/s at 40 °C, within ten percent. That is a laboratory reference point, not an operating condition, and confusing the two is the most common selection error.

What the pump cares about is the viscosity of the oil at the temperature the oil actually reaches in service. Every pump manufacturer publishes an operating window — a minimum viscosity below which the film fails, an optimum band, and a maximum above which the pump cannot fill on the suction side. Vane, gear and piston pumps have different windows, and a system that runs at 65 °C in a hot shop needs a different grade from the same pump running at 35 °C.

Both directions of error have a signature:

• Too thin: metal-to-metal contact under load, wear at the pump and valves, and internal leakage past clearances. The leakage itself generates heat, which thins the oil further — the fault accelerates once it starts.
• Too thick: the pump cannot fill its inlet, which shows up as cavitation, noise and erosion damage. Power consumption rises and so does temperature, from fluid friction rather than leakage.

When a high viscosity index earns its cost

Viscosity index measures how much the viscosity changes with temperature. A higher number means a flatter curve. It is not a quality ranking — on a system that runs at one steady temperature all year, it buys nothing.

It buys a great deal on a system that does not. Two grades from our own published range, both nominally ISO VG 46:

At the reference temperature they are the same oil. At 100 °C the HVLP is roughly twenty percent thicker, which is the margin between holding a film and not holding one on a hot afternoon. At the other end the pour point is eighteen degrees lower, which is the difference between a machine that starts on a winter morning and one that stands there loading its pump against treacle.

So the test is simple: does this system see a wide temperature range, either through the seasons or between cold start and full load? If yes, HVLP. If it lives at a controlled indoor temperature, HLP and the money stays in your pocket.

Zinc-free, and when you actually need it

The standard anti-wear additive in hydraulic oil is zinc dialkyldithiophosphate — ZDDP. It works by forming a sacrificial film at points of metal contact, and for the overwhelming majority of systems it is the right chemistry.

Ashless alternatives exist because ZDDP has three known awkward areas. It can be aggressive toward yellow metals — bronze and brass components — particularly at elevated temperature. Its decomposition products can contribute to deposits in servo and proportional valves with fine clearances, and can load fine filtration. And it leaves ash, which matters where the fluid may reach a hot surface or where ash content is specified.

Our zinc-free grades are ashless rather than simply de-zinced, and they are matched on viscosity to the HLP range so that a change does not become a viscosity change at the same time: HLP ZF 46 measures 46.00 mm²/s at 40 °C and 6.650 at 100 °C, identical to HLP 46. The one figure that does move is the flash point, 216 °C against 230 °C.

• Bronze or brass in the circuit, especially where it runs warm.
• Servo or proportional valves, or filtration below about 10 µm where deposits are a known nuisance.
• An OEM specification that calls for ashless or zinc-free chemistry — in which case the decision is already made.

Getting it right in four steps

• Take the pump manufacturer's viscosity window — minimum, optimum, maximum. Not a general rule of thumb.
• Measure the reservoir oil temperature at normal running load, in the season that represents the worst case.
• Pick the ISO VG that lands inside the optimum band at that temperature. If cold start and running load pull in opposite directions, that is the case for HVLP.
• Check for bronze, servo valves or an ashless requirement before defaulting to a zinc-containing grade.

Full published data for the HLP, HLP ZF and HVLP ranges is on the technical data page, including the values quoted above. Send us the pump details and the measured oil temperature and we will confirm the grade against the manufacturer's window.

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