I'm not very good at writing online, I'm sorry for coming across in a way that was so strongly against putting oil on the shellac. I should have done a better job, I'm sorry!
Putting oil on a machine's paint job when it's in good condition is a great idea! But if you have an area where the shellac has had water on it, the water tends to lift the shellac...I'm not sure if 'lift' is the proper word. If the shellac has lifted, and water got underneath the shellac, putting oil on it will make it much harder to fix the shellac finish. Maybe we need to see a photo of the ring? It sounds like the water wasn't on there long enough to get underneath the shellac, but you definitely don't want to scrape it off with a fingernail.