Alright...I called my Amsoil dealer tech while I was driving to work...
Clearly, this isn't the first time this question has been asked. Without knowing what specific car/engine/year, the answer wasn't specific to your car, but would apply.
There are a few issues at play. First, the required fluid is very low. As little as 375ml. The second is the PTU is located closed to other components that throw off heat. The bigger engines are "worse". So you have the PTU with minimal fluid being baked which wears/thins the fluid quicker. And since this is allegedly a lifetime fluid, it's not changed often enough and you run into your issue. Because of these factors, he said it's essentially what would be considered "severe duty" situation even when driven normally. He said later versions come with more fluid and a drain plug.
They do indeed carry a fluid for your application. Depending on which vehicle you have, you could get a couple drain/fills from a single quart.
Hope this helps.