What if the rubber cap falls into the oil filler?

It depends on the size and hardness of the foreign body.

If you accidentally fall into a foreign object when refueling, it is usually a very small thing, such as tin foil paper the size of a fingernail, or a small foreign object stuck to a glove. Small soft foreign objects fall in and do not need to be taken out, which will not affect the engine.

The circulation path adds oil from the oil port to the valve chamber, and then the oil flows into the oil pan along the oil return line. The oil in the oil pan is sucked up by the oil pump, rises to the valve chamber through the cylinder block, lubricates the camshaft, and then flows back to the oil pan.