I bought a 623 switcher that is basically a barn find - it is 60+ years old, and I swear it must have the original grease on it! The grease on the side frames is hard like dried clay and even more stubborn to get off. Soaking the parts in mineral spirits and then vigorously scrubbing them with a toothbrush got rid of the fresher layers of grease, but left the crud you see in the picture. I found that a small screwdriver blade and a new wire brush got the dried crud off, but it takes like forever and the wire brush removes much of the black oxide finish!
I tried double soaking the parts in mineral spirits and then alcohol, followed by a toothbrush, but neither of the chemicals are really touching this dried grease at all. What do you use on dried grease???