So… I’ve used M1 ATF in prior honda 4-speeds, and as
@ctechbob mentioned, the shifts are certainly hard. BUT, if you add lubeguard Black to it, at precisely the measured dose, the shifts return to normal, at least in the case with a DW1-spec 4-speed. It was eerily quieter, and I think I lost a full mpg.
my particular trans had a shift flare issue with it, and the mobile 1 + LG black allowed me to fine tune the shift firmness overall. I started with less than the recommended LG dose and then slowly increased it until a good compromise was reached between flare and harshness.
I think you could do the same, as long as you find an ATF with similar viscosity. a thicker fluid in my experience slows the process of each shift, so that’s not great. But - you’re sort of rolling the dice. there are risks with messing with ATF fluids. I did have a CRV transmission develop problems on maxlife, even though many folks swear to maxlife in honda ATs. One option… and this is where I finally landed in my accord, was largely Honda ATF, then just a quart of M1 atf and the smallest amount of LG black to make it all balance out. Drove it a long time that way, and at that point started adopting the “less is more,” if anything has to be done.
note - I would not try any of this sort of shenanigans in a Chrysler automatic. substitutions for ATF-3 in their 3 and 4 speed transaxles were known to cause the computer to compensate for the friction property changes in such a way as to prematurely wear out the clutches. There really are some risks in going too far off reservation.