I think I may have this same problem. I have a 92 LaSabre with 78k originial miles. A month or so ago I had my oil and filter changed at a quick change place. They came in and showed me my trans fluid on a paper towel and said it was bad but they didn’t recommend doing anything as that could cause the trans to fail. I never had a problem with the trans ever. Now a month or so later my trans fluid keeps dropping. I wonder if they could have done something to the inner seal you speak of? Is this inner seal difficult to replace yourself? Is it expensive? I would appreciate any more info. Thanks