Stubb, I soaked my TO-20 assembly in mineral spirits this weekend, and finally got it clean. (Kerosene works too, but I didn"t have any.)
I let it soak for a few hours, poured through the assembly (backwards), and repeated until the mineral spirits looked pretty clean.
As discussed in other threads, you can also drill out the spot welds holding the bottom screen on the assembly, then remove the screen and the filter media, which can be cleaned or replaced. (This site sells the filter media.)
Drilling out those welds, then riveting or screwing the screen back on so that the oil cup would fit, looked like a challenge, so I opted for soaking.