Since it was oil change time for the main gearbox I thought I'd give Syntrax a try to see if it helped reduce the really harsh rattling/clashing noises when the box was hot (previously been running engine oil - Castrol RX Super )
I know the clattering is pretty normal at idle (which I avoid), but I was also getting more than I expected when pulling away from around 12-1400RPM, and a LOT worse when hot.

I've only done about 100kms since the change, but it seems a fair bit quieter with the Syntrax in it (especially when hot), and much smoother to pull away from low-ish revs without the previous clattering.
Shift quality is pretty much the same , maybe feels a fraction less "notchy" if anything, but hasn't miraculously made it change like a modern sedan.
Maybe its just a case of new oil instead of the 6000km old stuff, or maybe I'm just dreaming - but I'm pretty happy with it so far.
Currently still running engine oil in the transfer as its been changed 3 times in the last 5000k's for speedo drive and CDL actuator work

Apart from trying to make it a bit smoother, the other reason I tried Syntrax was to fit in with the engine oil change intervals a bit better.
With the Isuzu twin filter/bypass I've got fitted, I'm planning to run 12,000km engine oil change intervals as was specified in the factory 4BD1T trucks, so didn't want to be having to drop the gearbox oil every 5-6K (and wasn't confident the engine oil in the g'box would last 12000k's).
I'll only be doing around 20K per year, so a couple of engine oil changes, and annually for transmission/axles should work out nicely.

Thanks also to whoever posted previously about removing the gear lever and filling the 'box through the opening. MUCH easier than trying to fill using the normal fill point.

Steve