Automatic chain oiler
The pump that continuously feeds bar and chain oil from the oil tank onto the guide bar while the saw is running. Most are driven off the engine so that oil delivery rises with chain speed, and many are adjustable.
An oiler that stops working is the fastest way to ruin a bar and chain, and the failure is usually a blocked oil hole in the bar or a clogged filter in the tank rather than the pump itself. Check delivery by running the saw at part throttle with the nose pointed at a light-coloured surface: a visible line of thrown oil means it is working. Turn the output up for long bars, hot weather and resinous timber. Use proper bar oil rather than used engine oil, which is thinner, lacks the tackifiers that make oil cling to a moving chain, and is a genuine environmental and health problem.