The designation API GL-4 denotes lubricants intended for axles with spiral bevel gears operating under moderate to severe conditions of speed and load or axles with hypoid (see note)gears operating under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications where MT-1 lubricants are unsuitable. The manufacturer's specific lubricant quality recommendations should be followed.
The designation API GL-5 denotes lubricants intended for gears, particularly hypoid (see note) gears, in axles operating under various combinations of high-speed/shock load and low-speed/high-torque conditions.
Source: API Gear Oil Specifications - oilspecifications.org