It's a bit lengthy, but I found this quick explanation on transmission fluid/gear lubricants RE: GL-4 and GL-5. Based on this, I think I'd better get myself a GL-5 oil! Vroom vroom.
b. API In 1965, API determined service classifications for gear lubricants based on their use. At that time, GL-1 through Gl-5 were issued, with GL-6 being added later.
API GL-1 is essentially a mineral oil for manual transmission and spiral-bevel axles plus worm-gear service satisfied by non-compounded oils.
API GL-2 is treated with very mild EP additive for automotive worm-gears not satisfied by non-compounded oils.
API GL-3 is treated with mild EP additive for manual transmissions and spiral-bevel axles operating under moderately severe conditions of speed and load.
API GL-4 is treated with medium level of EP additive suitable for lubrication of hypoid gears in moderate service. It is roughly equivalent to MIL-L-2105.
API GL-5 is treated with high levels of EP additive suitable for hypoid gears operated under high-speed, shock-load; high-speed, low torque; and low-speed, high torque conditions. This classification is usually equivalent to MIL-L-2105B
API GL-6 is treated with a very high level of EP additive for high offset hypoid gears (above 10 cm offset and approaching 25% of ring gear diameter) and other automotive equipment operated under high-speed, high performance conditions.
c. Limited Slip Differentials In addition to the above performance characteristics, gear lubricants going into limited-slip differentials service require the addition of a frictional modifier. A limited-slip differential allows the axles to turn at different speeds when on dry pavement buy will restrict differential action when on a slippery surface.
This system employs a slow-moving clutch which tends to "stick-slip" at low sliding velocities unless the gear lubricant contains the proper frictional properties. The "stick-slip" action produces loud chatter noises and severe vehicle vibrations.