On the Singer 500/401, often I pull the motor out if it is not turning at all. Then I drop in a bit of oil and turn the geared shaft back and forth until it seems to move easy. It does work in the oil. Some times those motors get dried up oil in there and they don't want to run - I've had a few that sounded like a Mack truck. If it isn't frozen up, put a drop of oil on the bearing on the motor shaft then run the motor with the clutch dis-engaged.
I've also seen that you will need to drop some oil under the bobbin carrier and run it pretty good. I even tilt the machine forward and back - some times lay it on it's side to work in the oil. Be careful nothing hits on the table, your lap or where ever you work.