Trans fluid change on my 2007 suburban with 320,000 miles.

Just realized my post said 2007 ! Typo it's a 2009 6 speed. My bad!

Just be sure you can get the fill plug open before you drain it. Don't ask how I discovered this nugget of knowledge.

IdaD wrote:
Just be sure you can get the fill plug open before you drain it. Don't ask how I discovered this nugget of knowledge.

:B:B:B
LOL, learned that one the hard way myself years ago!!

Some mis info here, but good advice on the fluid change.
Honestly it’s flip a coin. I would change it. If it really is original trans with those miles it doesn’t owe you anything and I’d be surprised to see it last much longer.
If I wasn’t towing and just driving it around town I’d be apt to leave it alone but for work duty or long trips, you’re gambling with those miles anyway. Might as well give it the best chance for survival.

Experienced this first hand...bought a used 2000 Honda Odyssey with about 130,000 miles on it.

Had advised anyone who would ask/listen to flush their automatic.

Generally no problems...but...there is a chance to loosen or dissolve deposits (varnish is the main thing) on precision mating/sliding/etc parts. That will either float to make something stick and/or the mating/sliding parts no longer have the precision needed to do their jobs

On a trip to visit grandson in S Calif and loaded with tools to help SIL remodel their garage...coming back running down "The Grape Vine"...noticed the tranny shifting 'funny'. Lethargic both up-shifting and downshifting on the grades. After getting home...it continued to shift weird. Had the tranny's ATF flushed before the trip and all seemed fine for the weeks before that trip.

Mechanic said Odyssey's automatic is the weak system and he rebuilds tons of them. Mostly folks who load up to GVWR and/or tows. $3K cost and now know the automatic is the weak link from experience...

He says they tend to over heat the ATF and he has to clean the varnishes, but replaces valve bodies as that is where the main issue resides.

Now have an in-line ATF filter and he says to put in an after market ATF cooler...but there isn't enough room for that. At 18 years old...am considering getting another used Odyssey and think sooner than want to...

Suggest the OP just do the pan, filter and leave the majority of the old ATF in there....as long as it isn't too dark...nor smells of burnt ATF...do the white tissue test...

On high mileage trannies & ATF...it is 50/50 chance of creating a problem...or better stated...unmasking a potential failure now rather than it doing so by itself...