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Guide

Hydraulic Oil – Basics, ISO VG and Selection

What hydraulic oils do, which ISO VG viscosity is right, and what to consider with HLP, HVLP, HLPD & more.

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Hydraulic Oil

What is Hydraulic Oil?

Hydraulic oil is the working medium in hydraulic systems. It transmits power, lubricates pumps and valves, dissipates heat, and protects against corrosion and wear. Applications include industrial plants, presses, injection molding machines, construction machinery, and agricultural equipment.

  • Pressure transmission with defined friction values
  • Excellent lubrication and wear protection performance
  • Good air release and anti-foam properties
  • Thermal stability and oxidation resistance

In Brief

Always select viscosity and type according to manufacturer specifications – especially for mobile equipment and wide temperature ranges.

Viscosity: ISO VG & VI

Common grades: ISO VG 22, 32, 46, 68, 100. The Viscosity Index (VI) describes temperature behavior.

Stationary Systems

Often HLP 46/68 with moderate VI – constant temperatures, robust additive package.

Mobile Hydraulics

HVLP with high VI for wide temperature ranges and cold start capability.

Special Cases

HLPD for water/contamination, Bio-oils (HEES/HEPG) in sensitive areas, HF fluids for fire protection requirements.

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Specifications and Types

Important standards and classifications:

  • DIN 51524: HL (base), HLP (AW/EP), HVLP (high VI), HLPD (Detergent/Dispersant)
  • ISO: ISO 11158 (HL, HM, HV, HG), Cleanliness per ISO 4406
  • OEM: e.g. Bosch Rexroth, Denison HF-0/HF-1/HF-2, Eaton Vickers
  • Fire-resistant/Bio fluids: HF (HFA/HFB/HFC/HFD), HEES/HEPG (biodegradable)

Zinc-containing AW additives provide strong wear protection; zinc-free formulations are used for sensitive additive systems or environmental regulations.

Practical Tip

Document approvals and filter ratings for each system. This prevents mixing and reduces downtime.

Oil Care Consulting

Maintenance, Oil Care and Monitoring

  • Cleanliness: ISO 4406 target class per OEM; check particle count regularly.
  • Water Content: Keep low (consider demulsibility); vacuum dehydration if needed.
  • Filtration: Appropriate fineness and beta values; consider bypass/offline filters.
  • Oil Analysis: Viscosity, TAN, oxidation, additive condition, wear metals.
  • Air/Foam: Monitor air release and foam suppression capability.
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Avoid Common Mistakes

  • Wrong ISO VG/viscosity
  • Using HLPD unnecessarily (increased solvency)
  • Mixing zinc/zinc-free without approval
  • Insufficient filtration/oil care

Frequently Asked Questions about Hydraulic Oil

Quick answers for operators and maintenance personnel.

HLP or HVLP – what's the difference?
HVLP has a significantly higher viscosity index (VI) for better cold start properties and more stable viscosity across the temperature range – ideal for mobile hydraulics.
When do I need HLPD?
For system-related water ingress or contamination, e.g. in construction machinery. The detergent/dispersant additives keep contaminants in suspension.
Zinc-containing or zinc-free?
Zinc-containing provides strong AW protection, zinc-free is often an OEM requirement or environmentally mandated. Always follow OEM approvals.

Looking for the right hydraulic oil?

We support you with selection, oil care and approvals – for efficient, long-lasting systems.

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