Dot5 brake fluid or if you can get pure silicone oil, that's better. There is a thread on here where a guy went into some detail on this topic and testing and that's what he came up with. He seemed very pleased with the ease of cleaning after treating all internals with silicone oil. If ya can't find silicone oil, dot5 brake fluid has that in it and will work well.
I treated 2 of my new cans with it but haven't shot them enough to even clean just yet so