ahh, amc is showing his age, hehe.
It is true though, that even some newer-ish vehicles are still susceptible to that kind of problem. A buddy of mine had a late 90's Blazer that went about 60k on original tans fluid, and when he finally changed it, the trans had to warm up before it would lock into overdrive.
I've always kept fresh royal purple trans fluid in my getrag 6spd, and I can always tell when the fluid is starting to break down by how it shifts. I usually end up changing it every 1-2 years, depending on how much it gets driven. Well worth the cost imo!