I have been using Tri-X Pan (3 v-notches) which Kodak rates at EI 320. I cannot seem to find this film any more, but only TXP 320, which has a different notch pattern, but the same EI. Are these films interchangeable? Is the "old" Tri-X still made/sold? And, if it is not available any more, will I need to test the TXP 320 for film speed and development times as I did the old Tri-X? If anyone has any information or experience with this, please let me know. Thanks a million!