What about using something like this - to suck out as much as you can out of the dipstick hole and top off with new fluid?
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I have been kicking the idea around on my 2007 Tahoe with ~~69K miles.
I have read a few places you can suck out 4-5 quarts - it holds 11-12.
Do it every 20K miles -
I am chicken - afraid it will cause problems.
The service writer at the Chevy dealer says just don't do anything until 100K - then service it right with a flush and fill.
When I ask if that could cause issues he says NO we know how to do it correctly and use the correct fluid. Most issues are people know knowing what they are doing and then using the wrong fluid.