Smitty wrote:wrong viscosity
If you want other than 10W, yes. If need something just a tiny little bit thicker than 10W then Dexron IIIH is. Dexron VI is just a little bit less than the 32 centistokes that many 10W fork oils seem to be (at 40 deg C, ASTM D 445 etc).
Looking at spec sheets, it seems the Dexrons may have a higher viscosity index as well so their viscosity drops a bit less as they heat up.
Smitty wrote:wrong additives and composition
ATF does lots of hydraulicy vavley things in a tranny and will lubricate moving metal parts for a long time and at high temperatures.
Smitty wrote:sewing machine oil or similar would be a closer match to fork oil
I haven't seen sewing machine oil in the quantities required.