@Handyhubby - Dave, no 3-n-1 oil. You will really make your machine's life miserable. Stick with either sewing machine oil or TriFlow Superior Lubricant oil (preferred). As for recommended grease for gears, etc., I use Vaseline (petroleum jelly). I have read the currently available sewing machine grease is more suitable for newer - not vintage - machines.
You have a beautiful 15-91, enjoy it. I am in the last steps of cleaning up one I found too (wiring repair) and look forward to using it.
John Thomas in NC