Sounds like you did good! I would not, however, put oil anywhere inside the tension assembly. If it's gummy, I clean it up with a little WD-40. (Sorry, I know some people get up tight when I say that.) When you put the tension assembly back into the machine, it should push against the left side of the machine to be installed correctly. It's kind of spring-loaded.
You can use either side of the tension disks. There are two slots so you can use two threads with a double needle.
If you're using a heavier thread (for upholstery) you'll need to tighten the upper tension about as tight as it will go to get a good stitch. And if you're feeding it from a large cone behind the machine, the thread needs to wrap behind a spool pin to keep it from popping out of the tension.
That is my favorite mechanical machine of all time! I've worked on hundreds of them.