OK, but I encounter just the opposite, in a way.
When I played Nadal in the French Open, his top spin was 100%, as it is now on grass. In the French Open, I used the slice a lot, and almost always, his return did not have much loop to it when I had used slice.
Now on grass, I am using slice, but his return shots have very much more top spin than they did on clay (where I also used slice). So, for me, it is just the opposite as you say, at least in terms of the effect that the slice is having on his top spin.
If I did not use slice on either clay or grass, I suppose then that his shots, as you say, would be more in a loop on clay than on grass, but that is not what I am seeing, because I am using slice. I am not using slice on grass just because I want to, but I am using it to try to minimize his top spin, which for me is still a problem on grass, but it is not working as it did on clay. So...why is that?
Also, if slice worked well on clay for this but does not work well on grass, what else can I do, because the top spin is still bad on grass, at least for me.