Yes, we actually do 2 auto trans fluids, one is Dex II compatable (and is extraordinarily powerful in older boxes or newer boxes on their last legs), the other is a synthetic multifill, which is compatible with Dex II and onward, 95LE and so on. There is only one gearbox I am aware of that it won't go into, which is the Falcon ZF 6 speed, as Ford have not yet released their fluid requirements for this.
The fluid is expensive, but built the same way as all oils and will reduce gearbox running temperature. If it was a Ford gearbox or if it is highly strung then I would say no question pump it through, changing all the fluid. However Holden boxes aren't too bad with heat (particularly since you have put a cooler on- good work by the way) so the choice is yours to either pump through and leave it for a couple of hundred thousand km, or drop the pan changing around half of the fluid in the box- not forgetting the filter of course, probably not servicing again for 40-60,000km depending on the condition of the oil in the box now.
Prices- 5 litres $90
20 litres $346
I'm sure I can do free delivery on the 20 litre, I will just need to check.
By the way your cooler- is that bypassing the factory cooling tank in the radiator, or running through that then to the cooler?
Brad