This is probably the best video on the gear driven models like the 15-91 and 201-2, there are one or two others that are very detailed and good too. It's hopefully much more info than you need to get yours in nice condition. Among the pictures there should be something on how to get to the gears behind the hand wheel. You can use either a light teflon grease like Triflow or vaseline. Vaseline is recommended for the motor wicks, or maybe the special grease from the Featherweight shop. There are issues with the wicking action on modern synthetic greases and it's hard to find the original saponified grease Singer recommended other than huge bucket size for industrial purposes.