Lakebom,
The Atlas 10D dates from approximately 1936 and 1937, after which it was superceded by the 10F made from 1938 through 1957. The 10D was available with both babbit and Timken tapered roller bearings, although the babbit bearing headstocks appear to be more common. Both originally had oil cups of the press-in type with spring-loaded top covers. As Spiral-Chips indicated, grease is not a good idea for babbit bearings and although it is commonly used in most tapered roller automotive wheel bearings (where use of oil is impractical plus the grease all rotates with the wheel and gravity can help a little bit to keep it distributed), it is inferior to oil for the reasons given plus once used, you can't go back to oil without first pulling the spindle and thoroughly washing out all of the dried grease. The grease will prevent oil getting to the bearings and will in just a few years dry out.
"Zerk" is or was the name of a company offering machine lubrication system components. They may have been the inventors of the so-called "Zerk" fittings as well as the chuck for connecting the grease injection gun to them. The name came to be synonymous with the fitting, although it has fallen into dis-use in recent years. If one of the fittings has never had grease through it, I suppose that with the proper tip on the delivery tube of the oil can you could use one as an oil fitting. But fittings are available (usually flat press-in style) for the function that work much better. They also have a spring-loaded ball to seal the injection hole and because they are flat around the ball are much easier to use. To install the grease fittings, the PO (Previous Owner) had to tap the holes in the bearing caps. But if he did not enlarge the holes first, you can probably re-install the proper oil cups. If your machine has babbit bearings, remove one bearing cap at a time and remove any old grease from the hole. Don't lose the shim packs that should be present under both ends of the cap. If it has Timken bearings, you will have to remove the spindle, bearings and dust caps in order to clean out all of the old grease. No one has to my knowledge ever come up with a 10D parts manual. B download the one on the 10F plus if you have Timken bearings the Atlas headstock Technical Bulletin. Aside from the plate that the motor switch mounts to, the two bearing versions of the headstocks are substantially the same.
