For R120 they may have used the wrong oil (I hope he did not use EP90 or some such?)... or a cheap mineral oil I would think. The R380 box in the Defender shows VAST improvements in shift quality with decent synthetic oils. I just replaced my main box and T-Case oil with Castrol BOT402 (My T-Case takes the same oil, yours won't) which is synthetic and I convinced the local Autozone down to R50 for the 500ml bottle. So at R100/l that was a R600 job but well worth it.
Most recommended box oil in SA for our conditions is the Castrol VMX for mineral and the MTF for synthetic but the MTF was replaced by the BOT402.
As to how often to replace, well, if you do lots of wet and muddy offroad work then maybe stay with the 'cheaper' oils and replace often, otherwise, get a nice synthetic for the main box. Makes shifting oh so easy... You can also get fancy synthetic oils for up to R180/l.
Your main box oil should be at least ATF Dextron 3 spec. I would check that invoice and make double sure they didn't fill with too thick an oil.
GU Patrol 3.0Di GL
Metalian Maxi in RED