2001 Allison 5-spd Auto maintenance question

[jocko]

Good evening truckers, I've got a basic question on fluid changes for the 2001 Allison 5 speed automatic (mine is behind an 8.1L). The owners and maintenance manuals call for fluid/filter service every 25k mi (for heavy use) and 50k mi (for regular use). As you know, the Allison has an internal filter and an external spin-on filter. Somewhere along the way I got the impression that it was appropriate to change the spin-on every 10-15k mi. Two questions have come up -

1) Why? I can't find any GM documentation that outlines changing the spin-on filter (only) at a lesser interval than the full fluid/filter service interval. Does anyone have any GM or Allision documentation that outlines whether/when to change the spin-on filter only?

2) IF it should be changed at lesser intervals - should the pan be drained when doing "just" the spin-on filter replacement? I would assume "yes" - but after consulting Dr. Youtube, only 1 of about 10-15 different vids on this topic demonstrate also draining the pan. It's a big difference in amount of fluid to replace in the system (and therefore, cost). It's only a few ounces of fluid to replace the spin-on filter but it's 6-7 quarts to replace the spin-on filter AND drain the pan. But the latter certainly makes a lot more sense to me - IF, that is, it's required at all. I've scoured the owners manual and the full set of GM service manuals and ATSG Allison 1000 manual with no luck (or I just missed it). The service manuals DO reference two different fluid replacement levels (7.4 qt for a fluid change or 12.7 for overhaul - i.e. accounting for diff in lines, cooler, and converter, etc) but only one service interval, so it's a little confusing. It never calls out doing "just" the spin-on filter - although it references the fluid change only amount. I noted that I only drained about 6 quarts when I changed the spin-on and drained the pan, so that's what I replaced and then did the hot trans level checks to top it off correctly. But I would like to get my arms around whether this is necessary. Of course it's better than not doing it - but given the look of the fluid I drained out after 12k mi, I highly doubt it was necessary at all.

Any insight appreciated, thank you.