the bolt on the top allows for replacing the filter there is a gasket(?) on the tube portion.the oil filler cap is a problem if not cleaned if you are in a dusty area and drive a lot.the cap allows are into engine and the tube allows it out .if filler is dirty you will get the problem you have.blowby is another cause easy to clean cap by soaking in varsol or similar type solvent then blow dry or let sit in upright position till dry.the filter was (is)a horse hair type material held in place with a mesh