The purpose of the dip stick is to check how much oil is in the engine. It has measurement markings on it. If you pull it out, you can see whether you have enough oil, or whether you need more by the level of oil on the markings. The oil filler cap is a plastic or metal cap that covers an opening into the valve cover. It allows you to add oil when the dipstick indicates that you need it.
Some cars have the crankcase vented through the filler cap.
Oil which is added through the filler passes down through openings in the head into the oil sump at the bottom of the engine. Oil Filter Oil filters are placed in the engine's oil system to strain dirt and abrasive materials out of the oil. The oil filter cannot remove things that dilute the oil, such as gasoline and acids. Removing the solid material does help cut down on the possibility of acids forming. With this type of filter, all of the oil passes through the filter before it reaches the engine bearings.