The manual for my Davis flat out tells you that if the *paraphrased* "machine is operating at less that optimal function due to gumminess from using poor quality oil" to drop kerosene into the oil holes, turn the handwheel several times, and then reoil with a quality sewing machine oil.
Speaking of the Davis, btw, this thing is GUNKY!! Can we say, oh, about 115 years of gunk that totally hid the fact that there are DECALS on the treadle base? Many of them are damaged/gone, but there are still quite a few left...even tho they tend to blend in with the black, I'm hoping the car wax I use on it when it's clean and the touch up painting is done will bring them out a bit more! I'll get some pics of what it looked like before, and what it looks like today (torn apart, etc.) on my blog tonight!