I remember more than once having a Festivia or it's replacement towed in with horrable trans noise (MTX) immediatley after an oil change. The clue to what had happened is that the dipstick showed the engine way overfull!!!
So I don't think you would be the first to make that mistake.
What do you think of the Fusion. I have not driven one yet. I keep hoping to get one as a rental car but it hasn't happened yet. I get a lot of GM stuff. Their stuff has really improved.