Too bad the archive search feature is so crippled; proper oil has been discussed a LOT in the past. It may be worth trying a search, anyway.
#20 is about ISO 49, if I remember correctly. Look at www.mscdirect.com or www.mcmaster.com and see what they have. You could also use hydraulic or turbine oil of the correct viscosity. They're about the same thing as "machine oil" and may be easier to find.
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