ATF in forks

[Mister_T]

Are there any good technicals reasons not to use Automatic Transmission Fluid ("ATF") in forks?

[Smitty]

.. wrong oil
wrong viscosity, wrong additives and composition

sewing machine oil or similar would be a closer match to fork oil

[Mister_T]

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.

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.

sewing machine oil or similar would be a closer match to fork oil

I haven't seen sewing machine oil in the quantities required.

[BrettZZR]

Lots of sweeping statements here, so I'll add...
There are differing ATF types as well - ATF220 is an extremely close match, and in some cases (AFAIK) is in fact the recommended oil. There is certainly no physical/mechanical problem with using it. My thoughts are that if there is any "loss of performance" in the handling department, most people (myself definitely included) don't ride well enough to tell the difference.
Me, I used a good fork oil - small quantity and long service life means any saving would be minimal.

[pkay]

In a lot of the old manuals they do specify ATF but I suspect back in the day grades and variations may have been a bit simpler.

Me I just use the castrol stuff from Super Cheap. They have 5w, 10w and 15w so you can play around until your heart's content. And it's not that pricey either.