It's a Hypoid gear oil.Great for differentials but not for transmissions.Hypoid gears are gears with curved teeth,i.e. pinion gears-especially those with differing axes.Look at a differential the pinion gear is not at 90 degrees to the ring gear.It comes in at a point higher or lower than that necessitating an oil with special properties.See below for a short explanation of the GL-5 rating.It'll work in a trans but isn't the ideal lubricant.
API GL-5, oils for severe conditions. They contain up to 6.5% effective antiscuffing additives. The general application of oils in this class are for hypoid gears having significant displacement of axes. They are recommended as universal oils to all other units of mechanical transmission (except gearboxes). Oils in this class, which have special approval of vehicle manufacturers, can be used in synchronized manual gearboxes only. API GL-5 oils can be used in limited slip differentials 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.