I don't think a mass produced NV5600 transmission is unique in itself, with different operating characteristics, based on which lube it is using.
However, I do think it might have unique operating differences based on it's owner/driver.
"NICK"
I tend to disagree - based upon the MANY reports of other owners, plus my own direct experience!
Specifically, I suspect the NV-5600's greatest weakness lies upon a poor synchronizer design that doesn't provide either the right material, or else insufficient surface area to properly engage sync action and properly slow gear rotation for smooth shifting between gears.
That "weakness", or design flaw makes these trannies unusually sensitive to the lubes that will work best in them for smoothest shifting. Sure, guys who try drag racing or otherwise "rush" shifting will see problems far sooner and more frequently than those who are slower and more careful - and they are also far more likely to prematurely wear out the syncros in the process. But they are only increasing the symptom and effect of that existing design weakness, not CAUSING it!
I suppose another way of expressing this weakness, is that the poor syncro function of these trannies creates a very narrow "window" of proper and acceptable shifting characteristic, and one that is quickly and easily tilted out of range by rapid shifts that prevent the weak suncros to properly adjust gear speeds, OR, lubes and internal variations in manufacturing tolerances that in their own way also prevent the needed syncro action in these specific transmissions -
but nevertheless, the real problem is FAR more likely a syncro design issue than a lube issue!
In my own case, my 5600 has always displayed the SLIGHT tendency when cold, to "catch" a gear between 2nd and 3rd - and on rare occasion from 3rd to 4th - but this can ALWAYS be reduced or eliminated by slower shifts - and goes away on it's own after a couple of blocks driving, or in warmer seasons.
BUT, on the 2 occasions I have experimented with "superior" lubes, in a vain effort to eliminate even the MINOR issues I had, it only made shifting FAR worse - and I suspect it was BECAUSE those lubes WERE superior, and rendered the already weak syncro action worse by eliminating the friction they needed to properly slow gear rotation for best shifting action.
AND, I have read enough here from other owners, guys who were very specific in stating that THEY TOO were shifting slowly and carefully - but still experienced grinding at shifts with whatever "better" lube they were using as a trial substitute.
So, that's my own theory, and while it doesn't provide all the answers or cover all the bases, in my own opinion, it is EXACTLY the variation in manufacturing tolerances of these trannies, combined WITH the weak syncro design, that causes rather wide variations in shifting smoothness from one brand/type lube to another...
