The small engines that run lawn mowers need their oil changed just like car engines. Most mower engines need their oil changed every 20-50 hours of operation.
A mower's owner's manual will indicate how often to perform oil changes on the mower. At the very least, an oil change should be done annually with other seasonal tune-up maintenance.
This article explains the steps for changing oil in most mowers. Changing mower oil is very simple and can take as little as five minutes.