Early engines used O-rings (also called packing rings or toric joints) as seals (first patented in 1896). These are just mechanical gaskets in the shape of a torus (a circular ring--like a lifesaver), seated in a groove and compressed during assembly between two or more parts. It creates a seal at the interface. However, O-rings require a fluid film to lubricate them. They have limited usefulness in vacuum application and at extremes of temperature. The modern oil seal represents a significant improvement over the simple O-ring because it effectively seals in lubrication and prevents contamination from outside under a wide range of pressures and temperatures.
Leather oil seals have great running properties and are mostly used for abrasive applications.
Rubber covered
Air side face The oil seal surface vertical to the center line of the shaft on the side that does not come in contact with substances to be sealed is called the back face.