Are you saying you can use Dot 4 with Code RSC?Dot 4 can exceed dot 5.1 standards. Dot 5.1 is sort of a gimmick.
Dot 4, 509 degree boiling point.
I think what you can do to avoid the water seeping in from the air is to put it in smaller bottles that are filled to the brim, after you are done bleeding your brakes. Then there is no air contact to let water in.FYI, whatever I can easily find, DOT 4 or 5.1 works well. Also, I've been finding that using tightly sealed, but previously opened cans (that have a tight top), have worked fine for me. That said I do probably bleed brakes more frequently than the average bear.
I'm sure I'll get burned by doing this some day, but I'll keep doing it till then to avoid wasting left over fluid.