Oil evaporates over time. The earliest machines came with two oil cans. one for oil, one for kerosene. The kerosene was used if the oil got dried up. It breaks down the varnish that oil becomes, so that you cen the re-oil them to restore operation.
I do not let my machines that are setup sit for more than 3-4 months without some form of use, machines that are packed down and stored get unpacked every 6 mos for rust check, wipe, and lube.