This fluid is trapped.

The stock Allison 1000 5 and 6 speed transmission pans have drain plugs yet it don’t permit a full drain. Unlike other transmissions, the Allison 1000 has two filters, one external and one internal. Allison recommends changing the external spin-on filter fairly frequently under heavy duty, or “severe” use, which is how we use our trucks. Their instructions recommend replacing the external filter at 50, 100 and 150,000 miles. They want fluid changes at 75 and 150,000. So, at 150,000 you’re replacing both the filter and the fluid. Then, you start the interval cycle again. They only call for the internal filter to be changed when the transmission is overhauled. In other words, you’re not dropping the pan unless you’re taking the trans down to the bare case. 

The new Banks Ram-Air trans pan has a magnetic drain plug at the lowest point, so the pan drains completely without removing it from the transmission. Not only is it convenient, it dramatically lowers fluid temps thanks to its patent-pending Ram-Air Scoop design. It’s now available for 2001-2019 Duramax and coming soon for 2020-2025 Duramax 10/1000, 2007-2018 RAM 6.7L with 68RFE, and Ford 6.7L with 6R140 and 10R180.