Kev, welcome aboard.
What you read was correct, the gearbox lubricant does indeed need to satisfy the requirements of a limited slip differential ... plus the spiral/bevel ring and pinion gears, bearings of all types, transmission gears and the synchronizer rings. The lubricant must also be non-corrosive to steel, aluminum, and "yellow' metals (i.e. copper, brass or bronze).
ZF's recommended viscosity was SAE 80 or SAE 80W90. The synchronizers, brass/bronze parts and limited slip differential within the gearbox has traditionally meant lubricants rated API GL-4 are the best lubricants for the ZF. GL-4 lubricants are medium load lubricants in which the additive package has been specifically formulated to meet the requirements of limited slip differentials and synchronizer rings, while being non-corrosive to aluminum and yellow metals. API GL-4 lubricants are common lubricants for transmissions and transaxles. The Castrol 80W90 Hypoy Gear Oil (GL-4) was among the lubricants ZF originally recommended to Ford.
GL-5 lubricants have a higher load rating than GL-4 lubricants, but the GL-5 additive package is usually not compatible with limited slip differentials or synchronizer rings; the GL-5 additive package is usually corrosive to yellow metals too. However, some API GL-5 lubricants contain friction modifying additives specifically added for limited slip differential usage (the label should specify it has a modified API rating such as GL-5+ or GL-5-LS). API GL-5 lubricants are common lubricants for axles that are independent from the transmission. Even when modified for limited slip differentials GL-5 lubricants are not usually considered suitable for the synchronizer rings & yellow metals in a transmission or transaxle. However in the case of Castrol's current 80W90 Hypoy Gear Oil (which is rated API GL-5) Lloyd's recommendation indicates to me it must be OK for the ZF. 
At least now you understand the issues.