I know that condition very well. That is slime flux, caused by a bacterial infection of the heartwood. There is no cure for the disease, and in fact the disease is very unlikely to harm the tree's health because it only affects the dead heartwood. The gases and fluids build pressure as the microbes ferment the sugars in the heartwood, and eventually that pressure works its way to the surface, where it is released as a slimy, sometimes foamy exudate. It often smells bad, and it attracts a variety of insects. If you have ever sawed an oak that smelled like dog urine, it was likely caused by a bacterial infection. The wood is often still perfectly usable, even decades after the infection begins, but eventually you'll get some other microbes in the mix that will rot heart of the tree.
