Thanks for your advice. I decided to go ahead and treat the inside of the frame since I already have the bike in my shop for repairs. There is some rust inside the seat tube at the very bottom and inside the BB housing. The upper part of the seat tube is shiny like new.

I went looking for Amsoil. I'm not going to pay extra for something relabelled for the bike industry. I found a Penzoil product in a similar spray can. It is for rustproofing internal parts of marine engines. I'm guessing it is the same thing as Framesaver. It was just over $3 for a 12 ounce can in the automotive department at Menards.