The question is raised,"how much fluid is too much, and how much too little"?

To much, and the fluid is picked up by the whirring bits inside the transmission and then entrains air and foams. Air entrained fluid pumps poorly, if at all. An over fill situation is not conducive to crisp performance, but there is probably lubrication enough not to cause problems. Surely all the moving parts get bathed in fluid.

How low a fluid level can be tolerated? Well, certainly not lower than the pick up tube, else the circulation pump goes wanting. We know there are both fluid lubrication circuits and fluid shift actuation circuits within the transmission. Both utilize the same fluid. ATF

Most of us are familiar with the sounds associated with pump cavitation due to a blocked fluid filter. Would low fluid levels also give an audible warning?
An additional risk of low fluid would be heat removal. There must be fluid enough to make it's way to any external cooler coils, fill the pump, and perform the functions of piston activation etc. Over heating of too small a volume of fluid could be a real threat. Oil does not tolerate high temperatures well. But at what level do things get critical? I doubt that even a liter deficiency (one of eight) would seriously effect transmission life or performance unless the service conditions were made miserible by high ambient temperatures, trailer towing, or overly spirited driving. Transmission abuse can take place under any fluid fill condition ;-) Fluid breakdown at high temperatures occurs at the surfaces of slipping clutches, but this process is made easier if the fluid arrives to this location already over temperature.

Perhaps the risk of poor service would be in the significantly underfilled condition that would happen if the fill process took place without the engine running and having the transmission cycled through the gears. All fluid passages should be filled all of the time, and THEN the fluid level be established.

I have no valid statistics regarding failure modes for our ZF 5HP24 transmissions. From this form, I have seen failed "baskets" and failed actuator "pistons". I have heard many complain of blocked filters. (myself included).
The "hunt and thump" is not acknowledged as a "failure" but does affect drivability, comfort and sophistication.
I don't see anything that suggests that these failure modes would be made worse by fluid changes. Or made better for the matter.
Additionally, I don't feel that the subtle fluid level changes due to small divergencies in the prescribed fluid temperature will result in catastrophic transmission failure.
It would be fun to fill a cold transmission to the overflow level, and then remove the plug a second time after running it for an hour to see how much oil was expelled. Hot fluid pouring over my hand is not high on my "fun list" however.

One must remember that the Audi Gods, in their wisdom, removed the transmission level dipstick because too many transmission failures were due to poor understanding of this indicator by the operators. Isuzu did this as well with the GM transmissions. Is "dip stickless" now standard for all automatics?


Lots to think about. Sorry if this bores you. I'll try to get over it.


Regards

Cal