Could someone have cleaned them with a cleaning product? A wood polish? Mr Clean?
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Is the oil fluid some form of viscous damping?
Rigid ceramic drivers are notorious for ringing like a bell - so they normally need some form of control (an ugly notch filter in the crossover or those rubber "dots" you see on accutron designs). Another approach is to use constrained layer damping and sandwich a soft layer between the rigid bell like layers: like a cake with soft gooey icing in the middle....is the icing leaking out perhaps?