You can refill either a cartridge from the non-disposable Lilly pen, or a disposable Kwikpen which has a built in cartridge. Either one. Here is the other thread:
filling-your-own-pens
If you fill it with 300 units and flush it as fully as you can each time, after the 3rd flush, this is the purity of the insulin you have in it:
0.9999997905950370000
Each time you refill it, it moves more toward completely pure. Of course it never gets to 100%, but when you have something like this, 0.9999997905950370000, I think you are in good shape.