Gear Oil ISO VG Selection Guide for Industrial Gearboxes

Gearbox failure is expensive. In Thai industrial facilities running 2–3 shifts, gearbox service life can vary by a factor of three between operations using the correct versus incorrect lubricant. This is not primarily about brand — it is about selecting the right viscosity grade and additive package for the actual application.

Understanding the ISO VG System

ISO VG (International Standards Organization Viscosity Grade) is the universal viscosity classification system, defined by ISO 3448 and measured as kinematic viscosity in cSt at 40°C:

ISO VG Grade	Midpoint (cSt @ 40°C)	Acceptable Range (cSt)
VG 68	68	61.2 – 74.8
VG 100	100	90 – 110
VG 150	150	135 – 165
VG 220	220	198 – 242
VG 320	320	288 – 352
VG 460	460	414 – 506
VG 680	680	612 – 748

Higher VG number = thicker oil = thicker film = higher load capacity, but also more churning losses and higher operating temperature.

Key Selection Factors

1. Speed and Load

General rule: Higher speed → lower VG / Lower speed or higher load → higher VG

Use the gearbox manufacturer's AGMA lubricant number or viscosity chart as your baseline, then adjust for operating temperature:

Application	Pitch Line Velocity	Recommended ISO VG
High-speed gearbox (direct motor drive)	> 10 m/s	68–150
Medium-speed (conveyor, fan, pump drive)	3–10 m/s	150–320
Low-speed, heavy duty (crusher, mill, agitator)	< 3 m/s	320–680
Worm gear (general)	—	220–460
Helical / bevel gear (general)	—	150–320
Open gear (no housing)	—	320–680 + EP additive

2. Operating Temperature — Critical for Thai Factories

Thai factories run 10–15°C hotter than European environments for which many OEM manuals were written. This means you typically need one or two viscosity grades higher than the manual specifies:

Estimated oil temperatures by facility type:

• Air-conditioned machinery room: ambient 25–30°C → oil temp ~55–75°C
• Open factory building: ambient 35–45°C → oil temp ~65–90°C
• Near kilns or boilers: oil temp can reach 90–110°C

Viscosity grade adjustment by oil temperature:

Typical Oil Temperature	Adjustment from OEM Spec
< 50°C	Use OEM specification as-is
50–70°C	Use OEM spec or go up 1 grade
70–90°C	Go up 1–2 grades (e.g., VG 220 → VG 320)
> 90°C	Go up 2–3 grades + consider synthetic

3. Mineral vs. Synthetic vs. Semi-Synthetic

Type	Advantages	Disadvantages	Best For
Mineral	Low cost, widely available, compatible with existing seals	Lower Viscosity Index, faster oxidation	General duty, scheduled oil changes
Semi-synthetic	Better Viscosity Index, moderate cost	Middle-ground performance	General to heavy duty
Full Synthetic (PAO/Ester)	High Viscosity Index, heat resistance, extended drain intervals	3–5× higher cost, may affect some older seals	Heavy duty, high temp, extended intervals

When to upgrade to synthetic:

1. Oil temperature regularly exceeds 80°C
2. Continuous operation (24/7) requiring extended oil change intervals
3. Gearbox OEM manual specifies "synthetic required" (common in newer SEW, Nord, Flender units)
4. High EP requirements, such as single-enveloping worm gears
5. Very low ambient temperatures (< 5°C) — uncommon in Thailand except cold storage facilities

GL-4 vs GL-5: A Critical Distinction

API GL (Gear Lubricant) classification defines the performance level of the additive package, particularly Extreme Pressure (EP) additives:

Classification	EP Level	Used For
GL-1	None	Spur/helical gear, low speed, light load
GL-4	High	Manual transmissions, industrial gearboxes with synchronizers or brass components
GL-5	Very High	Hypoid gears, axle drives with high pinion offset

Why You Cannot Substitute GL-5 for GL-4

This is one of the most common errors in Thai industrial maintenance: assuming that GL-5 is "better" because the number is higher.

The reality: GL-5 contains very high levels of sulfur-phosphorus EP additives, which:

1. Attack yellow metals (copper, bronze, brass) — Synchronizer rings, thrust washers, and bushings in many industrial gearboxes are made from brass or bronze and will corrode rapidly in contact with GL-5 EP additives at operating temperature.
2. Alter pH — Accelerate degradation of certain seal materials.
3. GL-4 is intentionally formulated to provide EP protection while maintaining compatibility with yellow metal components.

The rule:

• Helical, spur, bevel gears → GL-4 (the default for most industrial gearboxes)
• Hypoid gears, spiral bevel with high offset → GL-5
• Worm gears → GL-1, GL-4, or compounded oil per manufacturer specification
• Only use GL-5 when the gearbox manual explicitly requires it

Quick Selection Reference

Gearbox Type	Thai Factory Ambient	Recommended ISO VG	API GL	Notes
Helical, high speed (> 10 m/s)	30–40°C	VG 100–150	GL-4	Air-conditioned room
Helical, medium speed	35–45°C	VG 150–220	GL-4	Standard factory
Helical, low speed, heavy	> 40°C	VG 220–320	GL-4	Near heat sources
Worm gear (general)	35–45°C	VG 220–460	GL-1/GL-4	Varies by ratio and load
Bevel / spiral bevel	35–45°C	VG 220–320	GL-4
Hypoid gear / final drive	All temperatures	80W-90 or 85W-140	GL-5	Hypoid only
Open gear, large mill	> 40°C	VG 320–680	GL-5 or OG spec	+ adhesive requirement

Oil Change Intervals

Standard intervals (mineral oil, general duty):

• Enclosed gear unit: every 2,000–4,000 hours or 6 months, whichever comes first
• Oil temperature > 80°C: reduce to 1,000–2,000 hours

Warning signs to watch between scheduled changes:

• Oil turns dark black (oxidation) → change immediately
• Oil becomes cloudy (water contamination) → change immediately + inspect seals
• Metal particles in oil → inspect gearbox before changing oil

Synthetic oil: can be extended to 6,000–8,000 hours depending on application — confirm with oil analysis.

SK ZIC Industrial Gear Oil G-EP / G-FF Series

Sahawatthanakit (1988) supplies SK ZIC G-EP (GL-5) and G-FF (GL-4) industrial gear oils:

• G-FF 75W-85 / 75W-90: GL-4, for manual transmissions and industrial helical gearboxes requiring GL-4 specification
• G-EP 75W-90 / 80W-90 / 85W-140: GL-5, for hypoid gears, final drives, and differentials

Both series meet API GL and AGMA lubricant standards. Technical Data Sheets (TDS) are available for engineering teams requiring specification verification.

Contact Sahawatthanakit (1988) for product selection assistance, TDS documentation, or bulk pricing.