A Singer 201-2 and a 15-91 have a potted motor (there is no belt). At the bottom of the motor are two "grease pots" that wick lubricant to the motor (they literally have cotton wicks in there). When the motor gets warm, the grease melts a little and passes through the wicks to the motor. For this particular application, Petroleum Jelly is generally used.
A Featherweight or other Singer machines may use some other lubricants, such as those developed by the Featherweight people. I don't have know about those applications. It is not the same application as the one for the 201-2 and 15-91. I don't believe they actually recommend their product for the potted motors.
But, of course, it's your machine and you can do whatever you think is best.
bkay