I will tell you from first hand experience that oil and cotton balls will clean the machines. The milky looking stuff that's left behind is old shellac. You don't want to remove that as it's all there is protecting the decals.
Glenn has an excellent tutorial on repairing the old shellac to protect your decals.
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Strong or abrasive cleaners will take the gold out of the decals in a heartbeat. Nearly instant destruction. Don't do it. Just use the sewing machine oil and cotton balls or flannel cloth. Go slow and easy, do a bit at a time and when you stop getting brown icky stuff off, your done.
Joe