Oil analysis has long been practiced for aircraft engines and other airborne oil-wetted systems such as hydraulic systems and gear boxes in helicopters. Military organizations are perhaps the number one users of oil analysis testing and commercial aviation has a long history of oil analysis. A number of airlines throughout the world do their own oil analysis, but in many cases they send samples to commercial oil laboratories. Engine manufacturers also do oil analysis. There are also several commercial laboratories in the USA that cater exclusively to the airline industry.