So far as I know, there are only three distillation methods for oud oil: steam, hydro and CO2 extraction. The latter is seldom used save in low quality cultivated oud. The results are far from impressive, with a pasty, sticky, solid at room temperature wax as the end product. The scent is impaired by the extraction of non-resin particles along with the agarwood essence.
Steam distillation is widely used in Indonesia. I am unsure about the benefits of using steam, considering that the oil is subjected to temperatures above 300 degrees centigrade. Some of my distillers harbor an intense dislike for steam distillation when it comes to oud. Yet Borneo 3000, Borneo Kinam, Kyara Koutan, and so forth were all steam distilled. Given the controversy I've found among distillers regarding steam, I do not plan on employing this extraction method from here onwards.
Then we have classic hydro distillation. Simple chemistry: you boil the wood and the resin rises to the top; from there you funnel it into a glass vessel where it gathers over the course of several days, floating atop the water. This is the oldest, most widely used method in Southeast Asia and Assam. The original Oud Royale, Oud Mostafa, Thai Encens (1 and 2), and other oils were extracted via this method.
This is where distillation can get real high tech, with different material tubes for different steps of the process.... You can have, for example, a stainless steel boiler with copper tubes that the oil travels through; or a fully stainless unit; or a fully copper one; or a copper still with stainless tubes; or different material tubes for different parts of the process, such as the water traveling through copper and the oil through steel; or vice versa. The possibilities are endless.
With steam distillation, all you get is agarwood oil that was heated up to a certain temperature and then separated from the condensed steam, with the resultant oil potentially impaired due to the high temperature. In hydro distillation, the raw materials are in close contact with water for a period of several days. The water has an almost magical effect on the oil, changing its character dramatically depending on how long it stays immersed, what type of water it is boiled in, the chemical breakdown of the water itself, salt and mineral content, etc.
Believe it or not, whether you get a fecal, a fruity or a woody, a dark or a light, a leathery or a green smelling oud oil all depends on the water you use to cook the raw materials!