To find the correct answer or explanation, it may go beyond the scope of DIY persons, and ask at engineering level.
What's considered is the type of gear cuts, (angles ), synchro rings and oil seal types.
Even if one tries a step down to a lighter grade of gear oil, due to the design of the box, it may have to be Dexron in Australia and proper gear oil elsewhere.
I also read somewhere, that it's rumored Ford Australia insisted KIA motor Company use Dexron from a marketing angle. If it is marketing, then I hope to get to the truth of the matter on purely engineering reason.
If, your gearbox is making noises after using gear oil when it is suggested to use Dexron, then damage is already done. If everyone else uses Dexron and there have been no issues, then maybe it is what it is, Dexron in Australia for both Manual and Auto.
Remember the Mini back in the day, it uses Motor oil for engine, gearbox and diff cos everything is in one whole unit. When the engine oil was drained, below the crankshaft is the gearbox and diff.
I think it comes down to type of gears and angles of the gears being cut to suit the oil.
Someone with engineering knowledge may be able to offer a light on this trivia
