Motor
I had a motor on a 15-90 with windings that looked exactly like that. It was carbon dust from the brushes mixed with a certain amount of oil. I carefully cleaned with 99% alcohol and some electronic contact cleaner, changed the brushes, re-invigorated the grease wicks and cleaned the commutator. The motor works fine. I plan on undercutting the mica to see if I can get it to run a tad smoother, but it's powerful and quiet and really does work fine.
Be careful to not allow any solvent to touch the insulation on the windings, but otherwise, I felt free to spray the contact cleaner into the windings to get rid of the oily carbon dust.
If left as is, you'll get smells and maybe even smoke when the motor heats up and the old oil starts to burn. May even be dangerous. Probably is really dangerous. I would clean the oil out of it as best I could before plugging it in.
What a great gift, a wonderful machine in a beautiful and desirable cabinet. Contrats. Let us know how the repair works out.