What element is stored under oil?

According to the BBC, Group one elements are stored under oil in order to prevent reactions with water vapour and oxygen present in the air. Group one elements include potassium, sodium and lithium.

Potassium, sodium and lithium can be cut easily using a blade. These fresh cut surfaces appear shiny and silvery, but can turn dull quickly from the reaction with oxygen. Group one metals all react in a vigorous manner with water to form a metal oxide and hydrogen. They also react with oxygen in a similar way. When burned, potassium has a lilac-colored flame; sodium, when burned, has a yellow-orange flame; and lithium burns a red-colored flame.