When I started swishing 3 months ago, I had the same foamy thing happen very quickly. Now, it takes the full 20 minutes, and even then, it does not thicken up like it used to....!?
I still gag sometimes, but when I changed to Sunflower Oil, it seemed to be lighter, and I didn't gag as much. When I feel like I am going to, I simply spit out, and take some more oil for the alloted time.
I think Sesame Oil is a very strong pull, or at least it feels like it is.