Gearbox (zf 6hp26 auto) there are options for Fuchs 4134 (some members on forum even use 4400 with repordedly very good results, although that is not stictly speaking the recommended oil, don't know if it is better or just different from 4134). About R150 a liter. Ravenol 6HP from sosoil.co.za is also a possibility. There is also a liqui moly gearbox oil if you can find it... The price on the gearbox oils matters because the volumes for double flush are huge. The diff oils I'm still looking for alternatives, but might just consider the prescribed dealer oils, because volumes are small. The transfer case and rear diff have the most specific requirements due to their locking nature.
Ps, I report this since I'm researching this myself, but there are probably experts on the forum with much more experience on these matters.