Motorex is $110 for 4 litres though Chris and the gearbox takes 1.1L whilst the engine only takes 600mL. If you switched to ATF Dexon at however much it costs you could change the gearbox oil every 250K’s (half of the engine oil life) and still save a heap.
As mentioend I’ll give it a whirl, I may also experiment with a decent synth oil for the engine.