I recently bought a singer from the 50's and went looking for how to care for it. I found an instruction book online that said to oil those old machines daily. I don't know if yours would need it that often, but I do know that sewing machine oil evaporates so that it won't gum up the machine. That is why it's important to use only sewing machine oil. I would suggest oiling it more often, maybe every week if you use it a lot, or bimonthly if you use it less often.
I can't answer about the age of the oil, but would think that it would be fine to keep using it. I think it is good for a long time.