JoJoe, I don't have experience with or know much about oyster shell, but it seems to be an "extra thing" that might not be needed if you can find chx grit of the size and price you want to help with drainage.
DO NOT USE Play Sand as it is too fine for drainage and will contribute to compactness of soil. If what you are using is working, available and affordable, then just stick with that. That way you only have to worry about your expanding landscape planning and your planting and watering chores for the specimens you already have out there.
