Reply to "What oil to run in my ZF gearbox?"

The gearbox lubricant needs to satisfy the requirements of a limited slip differential, 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 the metals used to manufacture the various parts within the gearbox.

ZF's original recommended gearbox lubricants had viscosity ratings of SAE 80 or SAE 80W90, and an API GL-4 rating. Castrol 80W90 Hypoy Gear Oil (rated GL-4) was among the lubricants ZF originally recommended to Ford. 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.

Some guys have good luck with synthetic oils, others find the synthetics tend to leak (drip) from all the seams between castings and from the seals, in those cases the owners have returned to petroleum based oils, and the leaks stopped leaking.