I spent probably way too long trying to understand how it all works. The gist of it is that you're definitely guaranteed to end up with more "fresh only" oil if you use the "hole in the pan" method over the cooler lines. But is that 1% better utilization, or 50%? No idea.

But I dug enough into it to find how the oil flows through the ZF 6HP transmissions. The below are some pictures I took, some I dub up online, some are even ripoff AliExpress parts (in some cases the best view I could find), but for the purposes of this explanation it doesn't matter.


First, all the oil that goes from the sump uptake into the oil pump goes back out into the mechatronic unit for further distribution. The holes at the bottom are from left to right: oil discharge (into mechatronic), suction (or "re-discharge", mechatronic doesn't want more oil at this time), and intake (where the plastic uptake from the cover plugs in):

In some of ZF's documentation it's ambiguous whether oil that's unwanted at the moment is dumped into the pan for re-uptake, or whether it's "spun" in place within the oil pump. The correct answer is the latter. That's good news for the "hole in a pan" method of flushing.

Now, the body of the transmission with the mechatronic removed looks like this:

You can see the intake port there in the bottom-left, then "suction" and discharge. What's even more interesting are the various other ports. From ZF's docs:



So in the picture above this one the "to cooler" is the elongated hole to the top-right of the oil ejection port (the large one with the rubber sleeve). So if all you want to do is evacuate the cooler with the mechatronic off you should be able to pump fresh fluid (or perhaps blow compressed air) into that port.

Interestingly the "lock up clutch release" port may allow for draining the fluid out of the torque converter. ZF's own documentation has this:

But it isn't elaborated on. I found some forum posts, e.g. this one speculating on this point, but nobody seems to have tried it and reported back. That's a compressed air gun above.

Anyway, once oil gets into the mechatronic it's distributed around the unit by various valves and ports, and oil going to the cooler is only one of those paths. I found the best illustration of the overall oil flow to be this picture:



That shows the mechatronic opened and turned upside-down. Critically because the oil discharge port (remember, discharging from the pump, this is oil going into the mechatronic) is now at the bottom that oil's drained out, so you can clearly see the path the oil's taking as it enters the mechatronic. It's the large passage going from top-left to mid-right in this photo.

The right side of this photo is where the solenoids are, so as they actuate the various gears get put into motion. And on the left side of this photo is where the fluid management (among other things) happens.

These two diagrams from ZF's documentation show that in detail. Now the unit's been turned around again, so the solenoids are on the left, fluid management on the right. The "WD-V", "WK-V" etc. is in reference to the schematic I posted earlier in this thread.



Anyway, I haven't been working up to some grand point all this time, except to say that it's clear that oil going in and out of the cooler is only one of many side-passages. There will be oil that goes directly into the mechatronic that just goes to solenoids and drips into the pan, up into the various gears and then drops down and splatters into the pan area etc.

I hope some of that rambling is useful to someone.