Two years ago when I got sorta serious about starting to use my equipment again I checked all of the local and chain auto parts and hardware stores (including two NAPA) within about an 8 miles radius of home (roughly 20 places). None carry SAE 20 non-detergent anymore except Ace and all that they have are the little 3-IN-ONE cans which if you use very much per month would be frightfully expensive. So I ordered some from Enco. However, later it suddenly dawned on me that Bass Tool and Supply has all sorts of specialty non-automotive lubricants. So check for a machine tool supply house in your area.
Unless you have a wet sump or an oil circulating pump and reservoir, using any weight oil on open gears isn't going to work for more than an hour or two. I'm going to look into Pierre's Mobil suggestion.
And incidentally, one problem the late Atlas machines don't have is swarf getting into the headstock.
Robert D.