Do not use grease. It will plug oil holes in the bushings.
I am not familiar with your bike but earlier models call for EP 140 oil. If you cannot get EP 140 oil, I have had success with heavy oil additives such as Lucas oil additive. To fill you could drill a small hole and pump oil in with an oil can, then reseal the hole with a sheet metal screw and a nylon or fiber washer if it is a sheet metal frost plug. If it is thin plate you could thread the hole and seal with a small bolt and washer.
Better yet, get the correct parts, but use a small bolt instead of the original fitting.
Personally, I would assume that someone has tried grease sometime in the bikes life and pull the plug to have a look. If grease was ever used the lube holes will be plugged. If so, the only solution is to pull the swing arm, pull the bushings, clean everything up, replace any worn/corroded parts and reassemble.