Ovals are nice, but the round ones will work fine as well. I've seen the cupped washers with the soft material inside used on Nisi's high-end rims, but not always. My experience is that they dry up over time without any change in spoke tension. They seemed no better or worse than any other washers I used & rode on as far as staying true and preventing the rim from cracking. Doesn't matter since I don't know of any way to get them today.