My helpful son topped off my '98 with a quart of synthethic ATF. The other 4 quarts is normal 10W40 oil - do I have an immediate problem or can I wait for the next scheduled oil change about 2,000 miles from now?
I wouldn't delay changing the oil, but it shouldn't hurt the engine. The ATF has lots of detergents and will break up any sludge or build up in your engine. That could cause oil leaks and will make your engine much cleaner inside. Sometimes, a quart of ATF is purposely dumped in the crank case to solve a ticking lifter (it cleans out the dirt that may have clogged the lifter). But of course it is not recommended. Change the oil soon as you can, but there should be no permanent damage.
I'll go ahead and copy the post I put in your same thread in the TALK section.
ATF has been used as an engine cleaner, just like the Motor Flush you can buy at an auto parts store. Chances are that it has worked any gunk in the engine loose. It's not a horrible emergency but I wouldn't drive it too far.
Put some fresh oil and a new filter on. Just use some cheapo oil & filter. Then change the oil & filter again after a few days and use the stuff you normally use. That'll get any residual out and it'll be fine.
ATF is actually very similar to 5w20, one of the reps for pennzoil actually ran it in his car for 3000 miles to prove a point, and no problem, but yeah, change it