This from Wikipedia, FWIW.  The article credits the Red Line web site as a
source:

It is extremely important to note that EP additives which contain
phosphorous/sulfurous compounds are corrosive to yellow metals such as the
copper and/or brass used in bushings and synchronizers; the GL1 class of
gear oils does not contain any EP additives and thus finds use in
applications which contain parts made of yellow metals.

GL-5 is not necessarily backward-compatible in synchro-mesh transmissions
which are designed for a GL-4 oil: GL-5 has a lower coefficient of friction
due to the higher concentration of EP additives over GL-4, and thus synchros
can not engage as effectively. Also, transmissions which explicitly call for
GL-4 oil may have been designed around this lower concentration of EP
additives and thus may contain yellow metal parts which GL-5 will corrode.

API GL-4. Oils for various conditions - light to heavy. They contain up to
4.0% effective anti-scuffing additives. Designed for bevel and hypoid gears
which have small displacement of axes, the gearboxes of trucks, and axle
units. These oils are standard for synchronized gearboxes, especially in
Europe, and may also be recommended for non-synchronized gearboxes of US
trucks, tractors and buses and for main and other gears of all vehicles.
GL-4 oils may also be used in many limited-slip differentials.

API GL-5. Oils for severe conditions. They contain up to 6.5% effective
anti-scuffing additives. The general application of oils in this class are
for hypoid gears having significant displacement of axes, generally non
limited-slip differentials. They are recommended as universal oils to all
other units of mechanical transmission (except synchronized gearboxes
specifying GL-4). Some GL-5 oils in this class, which have special approval
of vehicle manufacturers, can be used in synchronized manual gearboxes. API
GL-5 oils can be used in limited slip differentials only if they correspond
to the requirements of specification MIL-L-2105D or ZF TE-ML-05. In this
case the designation of class will be another, for example API GL-5+ or API
GL-5 LS.

Karl Wolz