Engine generators had wicks going into the bushings before sealed bearings came along. According to the owners manuals, you would put a couple of drops of oil into the cups every ten hours of operation or so.
For a sewing machine, I would say a drop of sewing machine oil onto the wicks before you start a day of sewing would be sufficient.
If the wicks are dry, apply a drop of oil, then wait ten minutes to see if the oil has worked its way into the bushing. If it is still dry, add another drop of oil.