Because of the stresses that rototillng put on the chaincase of a tiller, it is very difficult to design them to be oil-filled. The seals on the tine shaft are easily compromised by dirt and trash. As such, many manufacturers just filled them with chassis grease to keep the chain lubricated. If you have dismantled the chaincase, then you should clean all of the old lubricant out, wash everthing in a solvent, install a brand new chain and then repack it with fresh grease during re-assembly.

Chains stretch and then break so if you have it apart, put a new one in and spare yourself that kick in the butt you will give yourself if the old chain lets go in the midst of a tilling operation.