Hi,
Firstly, welcome to the Forum.
50 SAE by Rock Oil, Warrington UK. For both primary drive and gearbox.
Ummm ... why?
Over half-a-century ago, your bike's maker (defunct nearly forty years ago) recommended "50 SAE" engine oil in the gearbox
only.
Triumph recommended much lighter
20 "SAE" in the primary, because it's for lubricating the primary chain when its bottom run dips in the oil; "50 SAE" is too thick to do this (which is why Triumph didn't recommend it).
Nevertheless, as I say, we're talking recommendations over half-a-century old. Some years earlier, Triumph stopped recommending acetylene for the lights because reliable electric lamps were available. A bit before that, Triumph stopped using hot-tube ignition because HT coils and spark plugs were more reliable.
Similarly, only a few years after they made your bike, without any major design or mechanical changes to the gearbox, Triumph recommended SAE EP90 proper gearbox oil in the gearbox - "EP" indicates Extreme Pressure additives, that engine oil doesn't have but specifically gearbox oil does.
Aside, at the same time, also without any major design or mechanical changes to the engine, Triumph changed to recommending SAE 20W50 multigrade engine oil. Even at the time, multigrades had been around for over twenty years and in common usage world-wide for over ten years, so Triumph was hardly at the technical bleeding edge.
One reason you had to buy "50 SAE" from half-a-planet away might be no-one in NZ is daft enough to waste money on it? But you can buy EP90 and 20W50 easily?
Finally here, I sincerely hope you aren't also using the tyre pressures Triumph recommended 53 years ago? You'll find the handling on any modern tyres very weird, to the point of being dangerous.
Hth.
Regards,