Just dump some oil in it. It isn't rocket science. The oil that was used in it originally was poor compared to what we have now and it lasted just fine. You aren't ever going to wear it out if you keep it full of oil and change it when you need to.
EVERYTHING on the farm gets Rotella 15-40. I haven't ever read the label to see if it is "recommended" or not. Too late now as this stuff has been running on it for years.