06 auto trans question

My guess would be the heat and the mess involved. Toyota in their infinite wisdom made an easy job difficult by eliminating the dipstick under the hood.

 

Disclaimer, i haven't done this. Take this for what it's worth...
I don't see why not. Aside from possibly stick the hose in too far and it getting chewed up inside. You have the level check open while it's running, assuming you're not a quart low, i can't see cold fluid added to a warm running transmission being an issue... I'd do it. Fill it till it starts coming out, let it die down to a trickle and close it up