Electric motors have bearings with permanent lubrication, requiring no additional lubrication. Compressors have their own oil supply (oil recommended by the manufacturer. For adding oil in installations with long gas lines, check the manufacturer's recommendations. The compressors have an oil sight glass for checking the level. The oil level must be checked when the compressor is running in stabilized conditions, in which case the oil level should be between 1/4 and 3/4 of the oil sight glass.