My old shop teacher once told us that if you were to rate transmission fluid using the engine oil viscosity scale, it would be equivalent to a 7w14 motor oil. There are a number of friction modifiers blended into transmission fluid that engine oil does not have so I would be leary of using a full load of engine oil in the trans. Maybe a pint or two for topping up in an emergency but not a complete fill. Can't say if the lack of those modifiers would make the trans more likely to slip or grab, but if this really was engine oil, I would suggest pulling it down and have a look while it's out of the vehicle.