For people who do not wear their automatic watch every day, watch winders are available to store automatic watches and keep them wound. This is particularly advantageous if the watch has complications, like perpetual calendars or moon phases. A watch winder is a device that can hold one or more watches and moves them in circular patterns to approximate the human motion that otherwise keeps the self-winding mechanism working. Older mechanical watches should be kept wound and running as much as possible to prevent its lubricants from congealing over time, which diminishes accuracy. Modern mechanical watches generally use synthetic oil; whether or not synthetic oils congeal is a point of contention among watch experts. A full service (which involves disassembly, cleaning and re-lubrication) should be performed at least every five years to keep the movement as accurate as possible.