Please read this warning, its very important.

If  your paintball air tanks fill nipple is leaking.  You must replace it.  DO NOT USE OIL, DOING SO CAN CAUSE A FLASH EXPLOSION AND YOU, THE PERSON FILLING YOUR TANK, AND BY STANDARDS CAN BE SERIOUSLY BURNED AND OR KILLED.

All HPA fill nipples have a little o-ring inside which eventually get warn out and will start to leak a little.  Typically in paintball when you have a slight leak if you put a little dab of paintball oil o-ring the leak will seal up.  NO OIL SHOULD BE USED ANYWHERE ON YOUR TANK INCLUDING THE FILL NIPPLE.

When your tank is filled the air going into your tank gets heated.  The Air gets hot enough to ignite the oil in the fill nipple and the oil which has migrated into the tank itself.  Once this oil get ignited, it has been known to cause a serious flash fire and burning people.  In extreme cases the paintball tank can actually explode. Do your self and the people around you a favor.  When your tanks fill nipple starts leaking spend $10 and replace it.  Please help keep paintball safe.