Differential oil

API Classes

Transmission oil (or gear oil) is classified according to the API (American Petroleum Institute) in GL and MT ratings.

• GL-1 - Deprecated: Used for mild conditions and low unit pressures. Contains oxidation and rust inhibitors, defoamers and pour depressants.
• GL-2 - Deprecated: Used for worm-gear-axles and for conditions of load, sliding and temperatures where GL-1 wouldn't sufficate.
• GL-3 - Deprecated: Used for spiral-bevel-axles in mild to moderate conditions.
• GL-4 - Used for spiral-bevel and hypoid gears with moderate speeds and loads.
• GL-5 - Used for spiral-bevel and hypoid gears with high speed and high torque.
• GL-6 - Obsolete: Designed for very high pinion offset.
• GMT-1 - Used for non-synchronized manual transmission like buses and heavy-duty trucks. Exceeds the protection requirements of GL-4 and GL-5.

OEM Specifications

• Hypoid gear oil
• API GL-5 or GL-6
• Viscosity: SAE 90

Recommendations

Generally speaking you want:

• Meet or exceed the OEM specification
• The highest possible flashing point
• The lowest possible pour point

Castrol Syntrax Long Life 75W-90

Flash Point, ºC (ASTM D92)	224
Pourpoint, ºC (ASTM D97)	-57
Density, @15ºC, g/ml (ASTM D4052)	0,858
Viscosity, @40ºC, (ASTM D445)	101
Viscosity, @100ºC, (ASTM D445)	15,4
Viscosity index (DIN ISO 2909)	161
API	GL-5

Mobil1 SHC 75W-90

Flash Point, ºC (ASTM D92)	202
Pourpoint, ºC (ASTM D97)	-54
Density, @15ºC, g/ml (ASTM D4052)	0,87
Viscosity, @40ºC, (ASTM D445)	102
Viscosity, @100ºC, (ASTM D445)	15,1
Viscosity index (DIN ISO 2909)	156
API	GL-5