Don't use paint thinner! There are other, thin, oils that will free the machine. In fact, there is even descented kerosene. I have seen it somewhere. I use TriFlow, bicycle oil for all my antique machines. Some free up stuck part with Kroil oil. Thinner will add nothing. Don't use penetrating oil unless you remove it and lube with a good sewing machine oil afterwards. Some use hair dryers for heat and GENTLY tap, tap, tap with rubber mallets.