What are the three performance classifications defined by DIN 51524, and when is each applied?

DIN 51524 defines three hydraulic oil performance classifications. Part 1 (HL): rust and oxidation inhibited — minimum anti-wear performance for low-pressure hydraulic systems (≤150 bar) with gear pumps. Part 2 (HLP): adds anti-wear additive packages — the standard specification for mobile equipment hydraulics (excavators, wheel loaders, agricultural machinery) at 200–350 bar system pressure. Part 3 (HVLP): high-viscosity index fluid (VI ≥150) maintaining viscosity stability from −30°C to +90°C — specified for equipment operating across extreme ambient temperature ranges such as Arctic mining and desert construction machinery.

What viscosity grades are available within the DIN 51524 HLP classification?

DIN 51524 Part 2 (HLP) specifies performance requirements for viscosity grades VG 15, 22, 32, 46, 68, and 100 per ISO 3448. ISO VG 46 (kinematic viscosity 41.4–50.6 cSt at 40°C) is the most common grade for mobile equipment hydraulics in temperate climates (ambient −10°C to +40°C). VG 32 is selected for cold-climate equipment where cold-start pumpability must be maintained at −25°C. VG 68 is specified for high-pressure, high-temperature industrial hydraulics. All viscosity grades within a DIN 51524 classification must meet the same additive performance requirements.

How does DIN 51524 HLP differ from HM grade specified by some OEMs?

HLP (DIN 51524 Part 2) and HM (ISO 6743-4) are parallel specifications for anti-wear hydraulic oils. HM is the ISO 6743-4 designation for the same performance level as HLP — mineral hydraulic oil with rust, oxidation, and anti-wear performance. An oil meeting DIN 51524 HLP also meets ISO 6743-4 HM at the same viscosity grade. OEM specifications referencing HM (common in Japanese and US equipment) and those referencing HLP (common in European equipment) are specifying the same performance level. Applying HLP oil to an HM-specified system is generally acceptable, subject to OEM approval.

What compatibility requirements does DIN 51524 impose on filtration media?

DIN 51524 Part 2 (HLP) and Part 3 (HVLP) oils contain anti-wear additive packages — typically zinc dialkyldithiophosphate (ZDDP) or ashless phosphate ester AW chemistry. Filtration media in contact with these fluids must be chemically inert to the additive package to avoid additive stripping (depletion of AW protection during filter passage) or media degradation (loss of structural integrity and filtration efficiency). Synthetic microglass media used in SYNTRAX™ lube oil elements is qualified for DIN 51524 HLP and HVLP compatibility. Cellulose media may show additive adsorption at elevated temperature (>80°C) with certain AW additive chemistries — a risk factor for operators using extended drain intervals.

Why is HVLP (DIN 51524 Part 3) important for mobile equipment operating in extreme climates?

Viscosity index (VI) measures the rate of viscosity change with temperature — higher VI means less viscosity change per degree. Standard HLP mineral oil has a VI of approximately 95–105. HVLP Part 3 requires VI ≥150, achieved by blending multi-grade base stocks or adding VI improver polymers. For equipment operating at −25°C, a standard HLP 46 oil reaches 800–1,200 cSt at cold-start — generating severe pump cavitation and extreme filter differential pressure. HVLP 46 fluid at the same temperature reaches 200–350 cSt — within the pumpable range without bypass valve opening. The VI improvement reduces cold-start filter differential pressure by 3–5×, directly affecting NFPA T2.14 structural requirements for element collapse resistance.

How does DIN 51524 classification relate to OEM hydraulic fluid approval lists?

Major mobile equipment OEMs maintain hydraulic fluid approval lists that require minimum DIN 51524 classification plus additional proprietary test requirements. Caterpillar's HYDO Advanced specification, Volvo CE's VCE 1 fluid specification, and John Deere's HY-GARD specification all require DIN 51524 HLP or HVLP compliance as a baseline, then add further tests (copper corrosion, filter compatibility, air release, foam stability) specific to each OEM's hydraulic system design. Using a DIN 51524 HLP oil not specifically approved by the equipment OEM may void the hydraulic system warranty even if the basic performance classification is met.

What is the significance of oxidation stability in DIN 51524 HLP oils for filtration system design?

DIN 51524 Part 2 specifies oxidation stability by a minimum hours-to-specified-total-acid-number-increase test (IP 280 or DIN 51554 oxidation test). Oxidation stability directly affects filter service interval: as oil oxidizes, insoluble varnish precursors — aldehydes, peroxides, and polymerized hydrocarbons — form deposits that progressively block filter media pores. Oxidation-accelerated differential pressure rise can increase filter replacement frequency by 30–50% in high-temperature hydraulic systems running beyond DIN 51524 minimum oxidation stability. Systems operating above 80°C bulk fluid temperature require oils meeting tighter oxidation stability requirements than the DIN 51524 minimum.

Can DIN 51524 HLP and HVLP fluids be mixed if both are within the same viscosity grade?

DIN 51524 does not permit intentional mixing of different additive formulations even within the same performance class and viscosity grade. Different manufacturers' HLP or HVLP oils use proprietary additive chemistries that may be incompatible — producing precipitates, filter media plugging, or synergistic depletion of AW performance. The only safe practice when changing fluid brands or formulations within DIN 51524 compliance is to completely flush the hydraulic system: drain, refill with new fluid, circulate through filter for a minimum of two filter volumes, drain, refill. Joint TAN and viscosity monitoring is recommended for the first 250 hours following a fluid changeover.

What role does water contamination play in DIN 51524 HLP fluid degradation and filtration requirements?

DIN 51524 HLP fluids are mineral-oil based and inherently repel water — but emulsified water accelerates hydrolytic additive degradation and oxidation rates. Water in HLP fluids hydrolyzes ZDDP anti-wear additives, converting them to insoluble zinc phosphate deposits. These degradation products precipitate as submicron particles that challenge filtration — particles below 1 µm are not captured by standard hydraulic filter elements with β₆(c)≥200 ratings. Water contamination also reduces dielectric strength, accelerating electrostatic charging on filter media. ASTM D6304 Karl Fischer titration testing is recommended for HLP systems at annual intervals or whenever water contamination is suspected from seal failure or condensation.

How does DIN 51524 fluid viscosity grade selection affect filter element differential pressure and Beta ratio requirements?

Filter element pressure differential at a given flow rate is proportional to fluid viscosity. A filter rated at 3.5 bar differential with HLP 46 at 46 cSt will produce approximately 2.4 bar with HLP 32 at 32 cSt, and approximately 5.2 bar with HLP 68 at 68 cSt — per Darcy's law proportionality. This affects both bypass valve opening behaviour and the NFPA T2.14 collapse pressure margin. Switching from HLP 46 to HLP 68 in an existing system without verifying that the filter element collapse pressure remains >10× the new end-of-life differential at VG 68 viscosity may reduce the structural safety margin below the NFPA T2.14 requirement.

Knowledge Center›Standards›DIN 51524

INDUSTRIAL STANDARD · Active (multi-part)

DIN 51524

Hydraulic Fluids — Minimum Requirements (HL, HLP, HVLP Classifications)

Hydraulic and lube oil performance classification for industrial and mobile equipment — engine lube circuits, hydraulic power circuits, and industrial gear lubrication.

KEY PARAMETERS

Rust and oxidation inhibited — basic protection

HL class

Anti-wear additives — most common mobile equipment specification

HLP class

High-viscosity index — wide temperature range applications

HVLP class

Required when changing lubricant brand or formulation

Compatibility check

01 /

Classification System (HL / HLP / HVLP)

DIN 51524 defines three hydraulic oil performance classifications. Part 1 (HL): rust and oxidation inhibited — basic protection for low-demand applications. Part 2 (HLP): adds anti-wear additives for pump and proportional valve protection — the most common specification for mobile equipment hydraulics. Part 3 (HVLP): high-viscosity index hydraulic oil with viscosity stability across wide temperature ranges, specified for equipment operating from arctic to tropical conditions where cold-start and hot-running viscosity stability is critical.

02 /

Relevance to Filtration Media Compatibility

Filtration media must be chemically compatible with DIN 51524 fluids — particularly the additive packages in HLP and HVLP class oils. Incompatible media can cause additive stripping (depleting anti-wear protection) or media degradation (reducing filtration efficiency over the service interval). SYNTRAX™ lube oil filter elements are qualified for compatibility with DIN 51524 HLP and HVLP class fluids, ensuring no media degradation or additive interaction under normal operating conditions. Compatibility verification is required when switching lubricant brands or formulations within the DIN 51524 classification system.

03 /

Relationship to ISO Cleanliness Standards

DIN 51524 defines fluid composition quality; ISO 4406 defines particle contamination cleanliness codes. Both apply simultaneously in lube oil and hydraulic systems — the fluid must meet DIN 51524 composition requirements while the particle contamination level must meet ISO 4406 cleanliness targets (16/14/11 for system-approach engine lubrication; 17/15/12 for hydraulic circuits). A correctly specified DIN 51524 fluid with inadequate filtration will still fail contamination targets; a correctly filtered fluid using a non-compliant lubricant will compromise component anti-wear protection.

COMMON ENGINEERING MISTAKES

Selecting HLP grade mineral oil for hydraulic systems requiring HM specification. HM fluids add anti-wear additives for high-pressure pump protection — substituting HLP in HM-specified systems increases pump wear.

Choosing hydraulic fluid viscosity grade based only on ambient temperature without considering operating pressure and pump type. High-pressure axial piston pumps require higher viscosity than gear pumps at the same temperature.

Mixing DIN 51524 fluid classifications (HL, HLP, HVLP) in the same system. Additive package incompatibility can cause foaming, emulsification, and seal degradation.

ENGINEERING REFERENCES

STANDARD

DIN 51524-2:2017, Lubricants — Hydraulic Oils — Minimum Requirements for HLP Hydraulic Oils, Deutsches Institut für Normung

Primary standard for anti-wear hydraulic oil performance classification; defines HL, HLP, and HVLP chemical composition and performance minima for industrial and mobile equipment applications.

STANDARD

ISO 6743-4:2015, Lubricants, Industrial Oils and Related Products (Class L) — Classification — Part 4: Family H (Hydraulic Systems), ISO Geneva

Parallel international classification system for hydraulic fluids; HM corresponds to DIN 51524 HLP — enables cross-reference of European DIN and international ISO hydraulic fluid specifications.

STANDARD

ISO 4406:2021, Hydraulic Fluid Power — Fluids — Method for Coding the Level of Contamination by Solid Particles, ISO Geneva

Cleanliness code standard applied simultaneously with DIN 51524 fluid classification; both apply in hydraulic systems — fluid must meet DIN 51524 composition requirements while particle contamination must meet ISO 4406 cleanliness targets.

TEST METHOD

ASTM D943, Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils, ASTM International

Oxidation stability test method used in DIN 51524 qualification; extended drain interval HLP and HVLP fluids must demonstrate minimum hours-to-TAN-increase under ASTM D943 or IP 280 protocol.

FREQUENTLY ASKED QUESTIONS — DIN 51524

DIN 51524

Classification System (HL / HLP / HVLP)

Relevance to Filtration Media Compatibility

Relationship to ISO Cleanliness Standards

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