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One thing I read gave me a little pause. For folks who wait a bit too long to drain/fill . . . I read that it may be best to leave 15% to 20% old fluid in the transmission as it contains clutch material that will help worn clutch plates from slipping. Apparently, some have slipping issues when all the fluid is completely replaced in a transmission that has a lot of miles or went too long to replace the fluid. So by leaving some old fluid in it, the clutch residue helps provide some grip.
Yeah - I heard this same story back around 2002, with my '97 Taurus. I had driven it hard up and down the East Coast for 4 straight years ('98 - 2002) and over 100,000 miles, never changing the transmission fluid. This vehicle has no drain plug ... have to remove the 24-bolt pan ... very messy. Anyway, I posted on the Ford forums and was told to only replace what was in the pan; *NOT* to completely transfuse it (because the old fluid was needed to keep it from failing, etc.). I didn't buy the reasoning and went ahead with the complete transfusion anyway (dropped the pan, pulled the lower cooler line, pumped it out until "low flow", overfilled by 6 quarts, pumped it out the cooler line *again*). This procedure worked perfectly: on the 2nd pump-out, I had multiple empty quart containers all lined up and my extension hose coming from the lower cooler line filling each empty container one after the other. Right on cue, while filling the last empty quart container, the black fluid turned to red! So I had performed a complete transfusion of the fluid!

Then I drove the car. The transmission shifted very smoothly, but kind of vaguely. About 400 miles later, the Check Engine Light came ON with a Torque Converter Code (1700-series, IIRC) "Excessive Torque Converter Clutch Slippage Detected". Now, I would've blamed it on the fluid transfusion - had it not been for the fact that this code had already appeared previously. Offhand, without looking at my records, I don't know how often ... but probably just once or twice.

Thinking I was now heading for a transmission rebuild, I got a few quotes for the work. But it took me several weeks to do that, during which time I was still driving the car, being very careful how I accelerated from a stop (so as to not "slam" the transmission going from 1st to 2nd gear). After a few months had gone by, I decided I didn't need a transmission rebuild, and proceeded to drive the car to 226,000+ miles. As for the code - it disappeared along the way, and didn't come back.

So the transfusion may have created a "temporary" situation, but it wasn't a "terminal" one.

That said, I'm also not advocating for changing transmission fluid every 100,000 miles, although that's pretty much what I did with my Nox (LOL!). I really think the health of your cooling system plays a part in the longevity of your transmission fluid. My Gen-3 Taurus suffers from the notorious cooling system 'gunk up'. Bad Heat Transfer Fluid (coolant/anti-freeze) is going to result in burned transmission fluid. If you keep your cooling system fresh, I think you probably *can* change the fluid less often (at least on the vehicles where there's a pan, making it a more difficult job).
 
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There is a GM transmission oil dip stick, GM no. 2422-5190 and named "Indicator ASMTRANS Fluid LVL". Anybody tried it--looks like it will fit in my 2010 4-cyl just fine. GM Parts Giant shows it as a fit for my Nox.
 
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