DO NOT put oil in the motor, it will ruin it. Use only motor grease. I sell and service Featherweights in my spare time and I use Singer Motor Lubricant. It comes in a tube with a tip that makes it easy to insert the grease into the motor. I use a synthetic grease on the gears (if it has set a long time they probably need greased) You can buy a reproduction book for the featherweight or you can go to the Singer site and download a manual for free.
Congratulations on your find. Enjoy, they are a great little machine.