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I don't see where leaving old fluid in a transmission aids the longevity of a transmission.

It's not the fluid.

In the old days the proper way to "flush" a tranny was drain out the fluid you could. That would be about 60% to 70% of the fluid, the rest trapped in the torque converter. You would then refill. Run the car, row through the gears, and redrain. Refill, repeat. You would do this about three times. By circulating the old fluid out of the torque converter into the new, and then draining, you would eventually get about 95% of the old out, and the last 5% was fine.

That was how it use to be done - by the book.

Now when the flush the transmission they do a high pressure flush. A machine is connected to the tranny and blasts fluid at high pressure through the system, flushing out the accumulated fluid in the torque converter. The problem is after 100K miles it also, due to the higher pressure, stirs up any crap that might be in the tranny. That crap then gets into the solenoids, gears, and other hardware. The fluid "flush" is complete, you drive off and 3,000 miles later the tranny blows up.

This is why when shops do a flush they have you sign a waiver, saying that any tranny failure post flush isn't their problem.

The hard part is finding a shop that will do the old fashioned way, and do it right.

So the issue isn't leaving the old fluid behind. The issue is the tiny metal shavings and gunk floating around after 100K miles getting forced through your tranny, jamming up the machinery and resulting in a failure.

This is why there is a school of thought that at 100K miles, forget it, don't touch it you're better off.

Here is what I believe, for what it is worth. What does the owner's manual say? If the owner's manual says you should have flushed at 50K miles - then I say leave it alone. You could do more harm then good. If the owner's manual says 100K miles or further, then you're golden, flush away. If the owner's manual says it is a sealed system that requires no fluid replacement, I call shenanigans and say flush at 100K miles.

As I noted in a previous I reply I would then flush every 50K miles, regardless of what the manual says - and I would replace the tranny filter every 100K miles. Unlike an oil filter, the tranny filter can go longer between replacements. The matrix within the filter is not as fine as the oil filter, and it is designed to catch more "big" chunks (big is relative).
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