I'll try to help,if you take out the bobbincase the part that's left in the machine is the shuttle,it spins around when the machine is sewing,it rides in a groove this is where the oil needs to be put,in the groove around the outside I always tell people this is the highest wear point in the machine so I'd say give it a drop every time you start to use the machine & then every 1/2hr of sewing just a drop.
Parts are hard to find for this so oil the whole thing often.
Bob