Can you do permanent damage to an engine by mistakenly using transmission fluid in the crankcase instead of motor oil?
This mostly has to do with quantity.
If you put 1 qt. of transmission fluid in the crankcase, it will not be a problem if you get it out within a fairly short period of time. On the other hand, if you filled the crankcase with transmission fluid, rather than motor oil, damage to the bearings would take very rapidly.
How much trans fluid was added?
As VDC said how much and how long? Driving the car for 3000 miles with half ATF is different than just driving it around the block once.
This sounds like it might be one of those stories where someone who has no business opening the hood has tried to perform a fluid change. I hope we get some more details.
Actually, this is an old mechanic’s trick to clean out oil passages in sludgy engines. Add a quart of trans fluid to the motor oil. But, you need to change the fluid after running the engine for up to 15 minutes or risk damaging the rings or bearings.