outburst of oil from the filler cap

Corrosion on battery terminals is a result of hydrogen gas being released from the acid in the battery.

If the corrosion is on the negative terminal, on the negative terminal, your system is probably undercharging.

If on the positive side, it is probably overcharging.

Either replace the battery or put on eye protection, gloves and breathing protection and use an old toothbrush with a mix of water and backing soda to clean it up.

You can add some oil or a protective spray to prevent the build up.