300 Saybolt universal seconds is the viscosity required (300 SUS) which cross-references to ISO 68. So you need an ISO 68 oil, most of which at that low viscosity will be hydraulic oils.
Go to a Farm & Fleet type store, choices will abound for that common viscosity grade.
However, having gone through a similar exercise for several older lathes & mills, any sort of gear or hydraulic oil will likely work just fine. We replaced the headstock and way oils in several lathes and mills with a common ISO 150 gear oil we stocked in 55 gallon drums.