The LS trannies have a mixed history. Many, including mine have required major service way before normal time. Most of us have had the service covered under warranty.
Mine failed around 14,000miles, I bought it used. The symptoms were occasional delayed shifting and occasional very very harsh shifting, I literally thought I had been rear ended a couple times.
The drain plug is NOT a drain plug. It is just a level checker and a fill port. You pump fluid into the port until some dribbles out the hole. There is a tube from the "drain plug" that rises up into the pan to the correct fluid level. You will never get the tranny empty by using the "drain plug" You have to drop the pan or suck it out. I have read there is a fill port on the side of the tail. This may make it easier to fill and watch the fluid dribble out the "drain".
When my tranny was repaired under the "customer satsifaction program" it required them to drop the tranny and disassemble it. They replaced the valve body and solenoid pack and other parts. The fluid showed evidence of having over heated, this is a symptom of the common problems with the tranny. Overheating fluid at 14K miles at least in my experience is unheard of except in cases of abuse.
Without dropping the pan for regular fluid changes, there is no way to examine the fluid for early symptoms of problems. Without a dipstick you can't catch anything until like the manual says "Take the transmission in for sevice when it starts to shift erratically" Duh, too late by then.
After my experience with this tranny and based upon other tranny shop recommendations on trannys in general, I will probably be taking my tranny in for a fluid change every 50K or less. I might even go to 30K or less since after about 16K miles from my"rebuild", my tranny is once in a great while bang shifting again, could just be my imagination. Most tranny shops recommend 12K fluid changes.
The reason some high mile trannies fail is 1) The dirt is what kept the tranny working, really 2) Coincidence since the tranny was getting ready to go anyway.
Fluid changes are really an opportunity to inspect what is at the bottom of the pan and catch problems early. I am afraid this tranny is one of those that it will pay to keep an eye on.
Good Luck,
Jim Henderson