Oil seals are used to protect shafts and bearings from ingress of dirt and foreign matter and egress of oil or grease. An oil seal generally consists of an outer circular metal part and an inner flexible member that does the actual sealing and is bonded to the metal part by chemical adhesive agents.

The side with the spring and recess always faces the oil you are trying to contain. IE: Crank seals on the drive side or primary side are meant to retain the oil level in the chain case. The seal is installed with the spring facing out toward the primary case.