I asked the second guy what do they do with the motors, and he told me that if the customer complains the motor is running slow or something, they will disassemble the motor and clean the brushes, add carbon to the brushes, or do whatever needs to be done to get it working well again. Run it at top speed for a long time until you hear the motor change its sound. Then its run itself clean. It might take 10 mins, but you will hear the change in the motor's sound when it's finally cleaned. The bobbin race should be oiled daily, ONE DROP is sufficient.
Singer says oil the machine daily if used continuously.
8 or more hours a day. If you use the machine that much, oil it a least once a week, otherwise, once a month or when you hear more noise than normal. Just use a drop or two in each oil hole, more just collects dust and makes a mess. Use the Singer book on the Model 221 as a quide on where to oil. It has a reprint of the singer info in the back.