(sorry, accidentally cross posted).
GM says NO NO NO to any service other than a pan drop. (Unless you feel this is an ulterior motive :lol:)https://static.nhtsa.gov/odi/tsbs/2012/SB-10062497-7690.pdf
The use of external transmission fluid exchange or
flush machines is not recommended for the automatic
or manual transmission. Use of external machines to
replace the fluid may affect the operation or durability of
the transmission. Transmission fluid should only be
replaced by draining and refilling following procedures
in Service Information (SI). Refer to Automatic/Manual
Transmission Fluid and Filter Replacement.
And even if you exchange 12 qts in a flush (using the cooler lines, no machine), since the trans in P has lots of circuits blocked off, you are lucky to exchange 1/2 the fluid.
And if you do flush, don't even think of doing it without a filter change. http://www.agcoauto.com/content/news/p2_articleid/238
I recently did a pan drop/filter and was thinking of doing a fluid exchange thru the cooler. Then I read people sell those Lexus and Toyota trans with 250K and never touched any fluid. Still run like new.
In the early 90s after hs I worked briefly for a neighbor in a busy NYC transmission shop. He said if everyone did a pan/filter drop at 30K he would be out of business. He also said never to flush, either you waste money or will damage the unit. Zero benefit. He also said you never exchange all the fluid.
He also said trans fail after flushes because of clogged filters. He has seen many. If fluid is that contaminated that you need to be replaced it you have bigger problems. Now, with the new fluids the interval may be 60-100K.