By mistake less than 1 cup of synthetic motor oil (5w/30) was added to ATF Oil. Have not started vehicle. What do I do? Do I do nothing or drive/tow car to have transmission fluid changed?

I personally wouldn't worry about it. One cup will be so diluted with the rest of the oil, it shouldn't cause any problems.

suction gun, we use them to correct trans fluid that is overfilled. If you added it, and then stopped, it is floating on top perhaps. you could draw it out and let it go in a clean pan looking for the color difference. I would dump my trans fluid if it was not correct, but it has a plug for that. everyone else has to pull the pan. Your spec is going to be some specific kind for BMW right? like $5 a quart? now is your chance to get it out of there, if it bothers you.

after circulating it, it would be contaminating many more quarts. it would also be in the clutches and get in the converter. many converters have a locking clutch after the 80's or so. I had one with a cup of water in it once, it slipped until it was serviced.

You could get a suction gun at a place that has a full service shop and return it after?

Appreciate all the responses. For now, we used a (clean/unused) turkey bastor to remove about the same of oil that was added, off the top. Can't really tell if the motor oil stayed at the top or mixed with the ATF Oil. We'll call BMW in the morning (non-holiday) for final recommendation(s). Thank you!! Happy New Year!

IT'S ALL OK, DON'T YOU WORRY ABOUT, THE AMOUNT OF MOTOR OIL IS NOT ENOUGH TO CAUSE ANY DAMAGE TO TRANS,RELAX AND ENJOY THE DRIVE.