Non-detergent machine oil - aka hydraulic oil. Has all the appropriate anti-corrosion additives as near as I could tell when I researched it a year or two ago. It is exactly what is specified for most lathe type machines. There are a few specific specified oils, but ordinary (cheap) hydraulic oil seems to meet or exceed all the specs I found. Certainly good enough for a hobby machine that does not see 40-100 hours of hard use per week (more like a maximum of 2-3 hours of easy use).