I use ATF. Not a problem. Most brands of Dexron are about 9wt, Dexron/Mercon tends to be only 5-6wt. The factory fork oil is 10wt.
You can drain it from the screws. But the fact that it has fork drains means it's an older bike. You may want to pull the fork legs and do a full flush if you find nasty-smelling, emulsified oil in there.