[Richard]

It's a pretty simple concept. Oil floats on the water. The water or electrolyte bubbles when the battery is charging. The bubbling causes a mist. The cap is supposed to condense the mist. The oil simply cuts down on the mist. The problem is the you must ensure that the electrolyte always stays above the plates. Allow it to dip below the plates and the oil will coat the plate rendering that portion of the plate useless