Hydraulic Oil ISO VG 32 vs 46 vs 68: Key Differences + Grade Selection Table 2026

Most industrial hydraulic systems in Thailand default to ISO VG 46 — but that does not mean it is right for every machine. Servo precision systems require VG 32, and high-pressure hydraulic presses need VG 68. Using the wrong grade causes accelerated pump wear, reduced efficiency, faster oil oxidation, and significantly higher long-term costs. This article interprets ISO 6743-4, DIN 51524, and ISO 3448 with a grade selection table based on actual machinery types.

ISO VG Classification — Understanding the System

ISO 3448:1992 divides industrial lubricants into 18 grades by their midpoint viscosity at 40°C:

Most hydraulic systems use VG 22–100. Gearbox systems use VG 150+.

Viscosity Grade Selection Flowchart

flowchart TD
  A[Hydraulic system] --> B{Ambient operating
temperature?}
  B -->|< 10°C cold| C[VG 32 or lower]
  B -->|10–40°C normal| D{Operating pressure?}
  B -->|> 50°C hot| E[VG 68 or Synthetic]
  D -->|< 100 bar| F[VG 32]
  D -->|100–250 bar| G[VG 46
International standard]
  D -->|> 250 bar| H[VG 68]
  C --> I{Pump type?}
  F --> I
  G --> I
  H --> I
  E --> I
  I -->|Servo precision| J[Select VG one step lower
+ HVI ≥ 140]
  I -->|Standard gear/vane/piston| K[Use grade from table]
  I -->|Old / worn equipment| L[Select VG one step higher]
  J --> M[Follow OEM spec
as the primary guide]
  K --> M
  L --> M

Performance Comparison at Real Operating Conditions

Pump efficiency drops approximately 1–2% for every 10°C above the optimal operating temperature — this matters significantly over the long term.

ISO 6743-4 — Classification by Additive Package

In addition to viscosity grade, ISO 6743-4 also classifies hydraulic oils by their additive characteristics:

• HH — Straight mineral oil, no additives. No longer used in industry.
• HL — Anti-rust + anti-oxidation. Systems without high-pressure pumps.
• HM (= HLP in DIN 51524 Part 2) — HL + anti-wear (ZnDDP). Current standard for industrial hydraulic systems.
• HV — HM + Viscosity Index Improver (HVI). Mobile equipment, outdoor use.
• HG — Anti-stick-slip additive. Slideway oil for machine tools.
• HEPG / HETG / HEPR / HEES — Biodegradable, environmentally acceptable fluids.
• HFC / HFB / HFA / HFD — Fire-resistant fluids (steel mills, spark-free environments).

The standard in Thailand is HM (HLP) VG 46. This grade meets:

• Anti-wear test ASTM D6973 (Vickers V104C pump test)
• FZG gear wear ≥ Stage 11
• Oxidation stability ASTM D943 ≥ 2,000 hours

Hydraulic Oil Degradation — Warning Signs and Causes

ISO 4406 particle count reference levels:

• 18/16/13 — Suitable for servo systems, high precision
• 20/18/15 — Suitable for standard industrial pumps
• 22/20/17 — Suitable for heavy-duty applications (mining, construction)
• Worse than 22/20/17 — Drain and refill + flush the system

Cooling and Filtration — As Important as the Oil Grade

A hydraulic system running VG 46 at its best requires:

• Heat exchanger — Maintain oil temperature at 40–50°C (optimal range; do not allow to exceed 70°C)
• Reservoir size — At least 3–5 times the pump flow per minute (allowing dwell time for heat dissipation and air bubble separation)
• Filtration — 10-micron return line filter + 3-micron bypass filter for servo systems
• Desiccant breather — Prevent moisture entering the reservoir through the breather cap

Oil service life depends on these three factors more than the viscosity grade itself. A properly maintained system with a good cooler and good filtration can run VG 46 for 3–5 years or more.

6 Procurement Guidelines

1. Specify ISO 6743-4 HM/HV + ISO VG + DIN 51524 Part 2 in the TOR — all three standards should be referenced together.
2. Anti-wear ASTM D6973 V104C ≥ 50,000 cycles — mandatory for equipment running more than 1,000 hours per year.
3. VI ≥ 140 HVI for mobile or outdoor systems — mineral VI of 95–105 is insufficient.
4. Compatibility test before switching brands — mix 50:50 + heat to 90°C for 168 hours; inspect for sludge or separation.
5. MSDS + COA as official documents — with full spec and lot number for every batch.
6. Oil analysis service — a supplier that provides complimentary laboratory analysis is a valuable long-term partner.

Summary

ISO VG 46 is the international standard for industrial hydraulic systems in Thailand — suitable for most machinery operating at 10–40°C ambient with pressure up to 250 bar. VG 32 suits servo precision systems or cold environments. VG 68 suits high-pressure systems above 250 bar or elevated operating temperatures. HM/HLP additive package is the current standard — it must pass ASTM D6973, FZG ≥ 11, and ISO 4406 particle count per system requirements. Drain interval is 2,000–4,000 hours, but annual oil analysis is more cost-effective than scheduled draining.

Sahawatthanakit supplies HLP hydraulic oil in VG 32/46/68 grades from SK ZIC, complete with COA and complimentary oil analysis service for fleet customers. Contact our team for advice based on your actual equipment.

Frequently Asked Questions

What do the ISO VG grades mean? Viscosity Grade at 40°C per ISO 3448. VG 32 = 28.8–35.2 cSt, VG 46 = 41.4–50.6 cSt, VG 68 = 61.2–74.8 cSt. Higher number = higher viscosity.

Can VG 46 be used in most machinery? Generally yes — it is the international standard for industrial systems at 20–40°C ambient. Default specification for Bosch Rexroth, Parker, Eaton, and Vickers equipment.

When should VG 32 be used? Cold conditions below 10°C, servo precision systems, high-speed applications. Do not use at pressures above 200 bar — insufficient film strength will cause pump wear.

When should VG 68 be used? Pressures above 250 bar, temperatures above 50°C, older equipment with worn internal clearances.

How important is VI? VI 95–105 (mineral) — viscosity drops significantly with temperature. VI 150+ (HVI/Synthetic) — stable across a wide range. Mobile/outdoor systems require VI ≥ 140.

What should the drain interval be? Normal conditions: 2,000–4,000 hours or 1–2 years. Severe conditions (above 60°C, water contamination): 1,000–2,000 hours. Run oil analysis before draining — particle code 19/17/14 + acid number below 0.5 = oil is still serviceable.

Order & Inquiries

Sahawatthanakit (1988) Co., Ltd. — Hydraulic oil and industrial lubricant supplier across Thailand, VAT invoices available.

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