Differential oil

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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 suffocate.
  • 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