Hey, @Prevailing Wind, if the Purina OM food keeps on working for you and the cats, that's great. But boy, I had a bad situation a few years ago with Purina food with a couple of my kitties (they were littermates), and I did a whole lot of experimenting and research to find a different food that would work. I landed on Halo Sensitive Stomach dry food, which turned out to be a Godsend. Yes, expensive, but it totally solved my cats' problem.
Just wanted to share because it took me a lot of time and $$$ to figure that out, but we've been happy campers ever since.
Good luck to you and the kitties.