I see you are in Australia so I don't know about brands. Yanmar specified Universal Tractor Fluid which is available from several brands, and will be sold anywhere that has agricultural customers. The original UTF is labelled to meet Deere 'JD 303' specs and that is all you need in a moderate climate. There are also later, broader-temperature spec fluids available now but Yanmar doesn't mention those in their 1970's manuals.
Here, UTF is sold in 5 gallon pails. (~20 liters). Your YM1500 should take about 3/4 of that.