The round gaskets that keep oil inside a motorcycle engine are usually known as oil sealers. They are designed to prevent oil leaks by sealing the gap between the rotating shaft and the stationary engine housing. Oil seals are usually made of a durable rubber material and have a spring-loaded metal ring inside to help hold them in place and keep them tight.
There are different types of oil seals, including lip seals, which have a lip on one side that presses against the shaft to form a seal, and rotary shaft seals, which have a sealing ring that rotates with the crankshaft, for example.
A typical set of engine seals consists of several pieces of seals that match the engine, for a specific vehicle model.