I don't know when it was found but here is some technical info.

Most lubricants for manual gearboxes and differentials are hypoid gear oils. These contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears.

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


That being said, I choose to use
BRAD PENN®
Multi-Purpose 'Classic'
GL-4 SAE 80W-90 Gear Oil

The SAE 80W-90 is specially blended using high quality PENN-GRADE® mineral base stocks and select additive technologies. It is designed to be used with “yellow” metallurgy (brass, bronze, copper) and other ‘soft’ metals used in synchronizers, bushings, thrust washers and other components typically found in classic manual transmissions and transaxles. In applications specifying API GL-4 quality lubricants, use of API GL-5 lubricants with higher extreme pressure additive activity/concentration can damage these ‘soft’ metal components.