Castor oil is one of several lubricants that mixes with methanol and has been used for many many years. It has good rust prevention properties because it has an affinity for metallic surfaces. When it sits for a long time it dries out into a hard crusty substance much like regular vegetable oils. It tends to look like rust as the heat and combustion residues turn it brown. Heat and a little methanol usually loosens it up, acetone would probably dissolve it faster and cleaner though. I just took apart a few new engines recently that felt like they had sand in the bearings. Soaking in methanol for half a day broke most of the oil residue and had the bearings turning smooth as butter. The engines I speak of were factory run only 15+ years ago.