I like the VanSon ink that you can get from offset litho supply stores. Get their rubber based ink. But there are plenty of other inks to use too. You should order some Canada balsam (from Artcraft) to get more tack--it effectively hardens, but also makes the ink more sticky. The main thing to remember is to start with hard ink and soften it as necessary. But the way I usually mix, I get some soft first, then tack it up with the balsam. Once you get the soak time right, you can apply hard ink, then resoak and apply slightly softer ink, and you can use a brush charged with hard ink to remove soft ink in the high values.