Singer motors for the 99s use grease for lube, not oil. If you can't find the correct Singer motor lube in the tube, it's not the white stuff currently sold by sears, then get a craft syringe and use petroleum jelly. Inject the lube into the grease tubes at each end of the motor.
A belt that is too tight will cause the motor to over heat. It should be just tight enough so it doesn't slip. When tight enough it will look loose. This goes for a real V-belt, not the el cheapo rubber band belts.
The motor could also be dirty or oily inside and need the brushes inspected and cleaned.
Joe