Blumbly and folks,
Yes, of course we do have hotter ambient temperatures than the UK. And while this considerably affects engine operating temperatures, it has very little effect on those of gearboxes!
Another consideration is that the critical component, with respect to the correct oil viscosity requirement, in this model of Auto-Drive gearbox, is not the gearset. It's the multi-plate wet clutch assembly.
As it happens, I am very familiar with this type of clutch and its workings; most Japanese-made motorcycles use this type, and have done for decades.
BTW, I have the official Victa-endorsed Service & Repair Manual which covers this model, open in front of me as I type these words. That's Gregory's #424, p. 123-125, to be precise.
I would say that the reason that only the export Auto-Drives were actually marked with the oil viscosity grade, is that Victa dealers in the destination countries were very likely to be much further apart in distance, than they were in Oz at that time.
And Victa wanted to ensure that a critical maintenance procedure, i.e. regular gearbox oil changes, would not get deferred/neglected due to difficulties in obtaining Victa-branded oil.
This service item is flagged as essential in the S&R Manual...
