The reason transmission should not be overfilled is that the oil level
rises to the spinning parts which then fling it about aerating the oil.
So, what's bad about that? The transmission uses hydraulic pressure to
clamp its clutches and bands etc.; air compresses and the hydraulic
pressure can be compromised and so allow the clutches to slip during
Correct level is found with transmission hot, engine at idle, car level
and gear in Park. Level should be between the two marks on the dip stick
and not above or below that range.