Thanks so far for the input. There is something worrying me at the moment: I inquired about industry cleaners that are offered for such parts washers and found that these are TREMENDOUSLY expensive where I live.
What did you do? It's possible that such cleaners are cheaper in the USA, but probably not. These cleaners usually contain kerosene, is it possible to use normal kerosene which is probably cheaper, or does anybody have a special receipe for a cleaning mixture?