Hi Christian, 406 is a substitute for R12 just like 409. And R409 is equal to YH12. I use yh12 and it gives me very good results and the advantage of these substitute gases like the ones I mentioned with respect to r134 is that you don't have to change oil. There are many opinions regarding mixing gases, some say yes, others say that not then if it has very, very little original gas, you can throw 409 but it does not mean that it will break something but it may not yield the equipment as expected . If you do not have a vacuum pump or it is complicated to transfer it to the client's house, then take out the 406 almost to the limit of the minimum pressure inside the pipe and pour 409, and remember to throw it in liquid phase or with an inverted carafe. With the engine off it is ideal, but you need a balance and if you are going to start it with the engine on, add it slowly and slowly until the charge is complete, so that the compressor does not breathe in a liquid state and the piston valves or anything breaks. good luck greetings and value the answers