API Category GL-4 designates the type of service characteristic of spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications.
API Category GL-5 designates the type of service characteristic of gears, particularly hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions. Lubricants qualified under U.S. Military specification MIL-L-2105D (formerly MIL-L-2015C), MIL-PRF-2105E and SAE J2360 satisfy the requirements of the API GL-5 service designation.
As long as the GL-5 gear lube oil also says GL4 on the label its ok to use in GL-4 applications. In the past some equipment required GL-4 because GL-5 EP additves was not freindly to yellow metals used in the GL-4 spec'd gear case. The viscosity in not as important, single weight viscosities are a think of the past as most modern equipment spec's multi-visosity, e.g. 75W-90.