An electron or any molecule experinces electric force F = QE = 1.6 x 10-14 N approximately.The gravitational force F = ma = 9.1 x 10-30 N . Check out the power .You should ignore the weight of the electron or any charged molecule.
The oil drop has a weight cannot be ignored. The amount of charge the oil drop has is also very large and hence both these forces must be taken into account. If the oil drop is moving is also has drag forces . So the case of the oil drop becomes rather complicated . Upthrust cannot be ignored for more accurate calculations.
