An oil seal normally consists of three components: The Sealing Element (the nitrile rubber part), the Metal Case, and the Spring. It is a widely used sealing component. The function of a seal is to prevent the leakage of medium along the moving parts. This is mainly achieved by the sealing element. Nitrile Rubber. Oil Seals are Designed to aid trouble-free operation of your equipment, providing an excellent barrier against the escape of oils and greases.