Hydraulic Oil ISO VG 32, 46, 68 — How to Select the Right Grade for Industrial Machinery

The wrong hydraulic oil grade — even by one step — can destroy a pump within weeks. Yet on Thai factory floors, it's common practice to grab whatever is in stock rather than match the machine's specification. This guide explains how to correctly select ISO VG 32 / VG 46 / VG 68 the first time.

What Is ISO VG?

ISO VG (International Organization for Standardization Viscosity Grade) is a viscosity classification system for industrial lubricants defined in ISO 3448, measured at 40°C in units of cSt (centistokes).

The number after VG is the midpoint viscosity, accepted within ±10%:

Grade	Viscosity @40°C (cSt)	Accepted range
ISO VG 32	32 cSt	28.8–35.2
ISO VG 46	46 cSt	41.4–50.6
ISO VG 68	68 cSt	61.2–74.8

Higher number = thicker oil. Greater viscosity means better film strength under pressure, but also higher friction losses, greater energy consumption, and harder cold starts.

ISO VG 32 vs 46 vs 68 — Comparison Table

Property	VG 32	VG 46	VG 68
Viscosity @40°C	32 cSt	46 cSt	68 cSt
Viscosity @100°C	~5.5 cSt	~6.8 cSt	~8.8 cSt
Optimal operating temp	0–45°C	10–55°C	25–65°C
System pressure range	Low–medium (≤200 bar)	Medium (100–350 bar)	High (≥250 bar)
Cold-start performance	Excellent	Good	Fair
Energy efficiency	Excellent	Good	Fair
High-pressure film protection	Fair	Good	Excellent

Selection Guide by Machine Type

Use VG 32 when:

• Precision machine tools: CNC machining centers, lathes, milling machines — low viscosity for fast hydraulic response
• Low-to-medium pressure systems (< 200 bar)
• Air-conditioned factory environments (ambient < 25°C)
• Small gear pumps, servo valves, proportional control valves

Use VG 46 when (the most common grade):

• General industrial hydraulic systems (recommended default if the manual doesn't specify)
• Presses, injection molding machines, die casting equipment
• Hydraulic forklifts and material handling equipment
• General servo-hydraulic systems
• Ambient temperature 25–40°C (typical Thai factory conditions)

Use VG 68 when:

• Heavy-duty hydraulic presses: system pressure ≥ 250 bar
• Older injection molding machines where manufacturer specifies HM 68
• High-temperature continuous operation (ambient > 40°C sustained)
• Hydraulic hammers, industrial compactors, heavy construction equipment

flowchart TD
  A["Check machine manual
Does it specify ISO VG?"]
  A -->|"Yes"| B["Always follow the spec
ISO VG = manufacturer requirement"]
  A -->|"Not specified"| C{"Max system pressure?"}
  C -->|"< 200 bar"| D{"Ambient temperature?"}
  C -->|"200–350 bar"| E["VG 46
Standard grade"]
  C -->|"> 350 bar"| F["VG 68
or HV 68"]
  D -->|"< 25°C"| G["VG 32"]
  D -->|"25–40°C"| E
  D -->|"> 40°C continuous"| F

Consequences of Using the Wrong Grade

Oil too thick (e.g., VG 68 instead of VG 32):

• Slow pressure response → machine acts sluggish
• Difficult cold starts — pump takes longer to prime
• 3–8% excess energy consumption
• Filter clogging accelerated

Oil too thin (e.g., VG 32 instead of VG 68):

• Insufficient film thickness → metal-to-metal contact → accelerated wear
• Pump cavitation noise at high pressure
• Oil temperature runs higher than normal — accelerates oxidation degradation
• Seal and O-ring leakage faster (thin oil passes through clearances more easily)

Oil Change Intervals

Condition	Interval
Closed system, normal temperature ≤ 60°C	Every 2,000–4,000 hours
Oil temperature consistently 60–80°C	Every 1,000–2,000 hours
Water contamination (cloudy oil)	Change immediately
TAN (acid number) > 2 mg KOH/g	Change immediately — corrosive to pump
Viscosity shifted ±15% from new oil value	Change immediately

Visual check: black or dark brown oil with visible sediment means it's overdue for a change.

Hydraulic Oil Available from Sahawatthanakit

SK ZIC HYDRO Series (Korean Full Synthetic Base)

• ZIC HYDRO 32 — DIN 51524 Part 1 (HL), ISO 11158 HM, Cincinnati Machine P-68
• ZIC HYDRO 46 — DIN 51524 Part 2 (HM), ISO 11158 HM, Denison HF-0, Vickers I-286-S, Eaton (Vickers) 694
• ZIC HYDRO 68 — DIN 51524 Part 2 (HM), ISO 11158 HM, Bosch Rexroth RE 90220

Available pack sizes: 1L · 4L · 18L pail · 200L drum Nationwide delivery — same-day Bangkok metro, freight to all provinces

Order and Enquiry

Not sure which grade your machine needs? Send us the machine model, brand, or manual excerpt — our engineering team will confirm the correct spec before you order.

• Phone: +66-2-096-2118
• LINE: @406rrgvm
• Email: info@sahawatthanakit1988.com
• Request a quote: Click here