Gear Oil Singapore: GL-4 vs GL-5 and the Right Grade for Manual Gearboxes

Gear oil selection is one of the most commonly misunderstood areas of fleet maintenance. This guide covers API GL classification, SAE viscosity grades, and why using GL-5 in a GL-4 gearbox can cause expensive damage.

API GL-4 vs GL-5 — The Critical Difference

The API GL designation describes the extreme pressure (EP) additive level, not viscosity. Higher GL number = more EP additives.

Grade	EP Level	Correct Application
API GL-4	Moderate EP	Manual gearboxes, transaxles, some transfer cases
API GL-5	High EP	Hypoid differentials and axles with high-speed/high-torque contact
API GL-4/GL-5 (dual-rated)	Moderate–high EP	Suitable for both — safest general choice if unsure
MT-1	Thermal/oxidative focus	Non-synchronised heavy truck gearboxes (rare in SG)

Critical warning: GL-5 contains sulphur-phosphorus EP additives at concentrations that corrode yellow metals (brass, bronze, copper). Most manual gearboxes use brass synchroniser rings — filling with GL-5 instead of GL-4 gradually destroys the synchros, causing difficult gear changes and eventually total gearbox failure. Always check OEM specification before substituting.

SAE Viscosity Grades for Gear Oils

Gear oil SAE grades use a different scale to engine oil — do not confuse them. Common grades in Singapore:

SAE Grade	Application	Typical Equipment
SAE 75W-90	GL-4 or GL-4/5	Light truck/van gearboxes, compact commercial vehicles
SAE 80W-90	GL-4 or GL-5	Medium-duty lorry gearboxes, rear axles — most common grade in SG
SAE 85W-140	GL-5	Heavy-duty differentials, rigid axles, high-load hypoid drives
SAE 90 monograde	GL-4	Older gearboxes without OEM multigrade approval

Industrial Gearbox Oils

Industrial gearboxes (enclosed helical, bevel, worm) use ISO VG-classified gear oils — different again from automotive SAE grades:

ISO VG 150 — light-duty industrial gearboxes, lightly loaded helical units
ISO VG 220 — medium-duty enclosed gearboxes, most common industrial grade
ISO VG 320 — high-load or high-temperature industrial gearboxes
ISO VG 460/680 — very slow-speed, high-torque drives; worm gear units

Singapore’s heat means industrial gearboxes run toward the upper end of their thermal design envelope. If a gearbox shows signs of overheating (discoloured housing, oil darkening quickly), step up one ISO VG grade and verify cooling.

Change Intervals

Manual gearboxes: 60,000–100,000 km or as OEM specifies (some sealed units claim lifetime fill — not recommended in Singapore heat)
Differentials: 50,000–80,000 km depending on duty cycle
Industrial gearboxes: 3,000–6,000 hours or annually — sample and analyse before extending intervals

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We supply GL-4, GL-5, and industrial ISO VG gear oils in pails and drums. Same-day confirmation on standard grades.

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Gear Oil Singapore: GL-4 vs GL-5 Guide