For decades all we had was the fluid and filter change option. The flushing machines came about when transmissions were down-sized for cost-cutting and problems became more common. When you are not having problems, a fluid and filter change will replace about half of the fluid, and that will get you plenty of new additives such as seal conditioners and detergents.
My experience with this is limited, but I can share that the greatest benefit to flushing the system is to clean out the transmission cooler. In fact, doing that is required to put the warranty in force for many replacement transmissions. There are special pumps made for that purpose that do not flush the entire transmission, just the cooler.
If someone tells you the goal of flushing the transmission is to get the grit and metal chips out, be aware if those are in the clutch packs, they have already chewed up the rubber lip seals and you have major problems. If a metal chip or speck of dirt makes its way into the valve body, a valve can stick and you will have shifting problems where it stays in one gear at the wrong times.
The other concern is varnish build-up in the valve body. That can cause a valve to stick, but that only applies to which gear the transmission goes into or stays in. Sticking valves do not cause slipping. We do not hear of varnish problems often because changing the transmission fluid periodically removes most of it, and the additives do the rest. A flush is usually preceded by adding a can of highly-concentrated detergent to dissolve that varnish. I think I would be more worried about what else that detergent is acting on than any varnish left behind.
Thursday, May 3rd, 2018 AT 2:11 PM