As 30-284 mentioned, stay away from a synthetic gear lube and go with a mineral based lube like Torco, Valvoline, etc.. Your ARB does not require any special additives so any GL-5 or GL-6 will be fine. On the synthetic gear lube thing, axle manufacturers have recently, surprisingly to a whole lot of them, discovered that the R&P gears run hotter with synthetic gear lubes. Currently recently changed their axle warranty to require a mineral-based non-synthetic gear lube to keep their warranty in effect. They were having an unusually high number of R&P failures in their axles and traced it to those customers having swapped out the mineral-based gear lube Currie loaded them with with synthetic gear lubes.