When oil and patience don't work...
Try a hair dryer to deliver heat to each of the likely (metal only) junctions in the stitch length mechanism path, turning the handwheel while heating up the parts. I would be tempted, then, after turning off the hair drier, to follow up with a chaser drop of oil at each joint/part of the mechanism that turns, rotates, slides, revolves, swings, etc. Keep turning that handwheel while you do so.