Is saying "By God, you are a donkey" considered a false oath, and what is the ruling on doubting false oaths?

It is not permissible to curse a Muslim or describe him as a donkey, for the Prophet, peace and blessings be upon him, said: "Cursing a Muslim is a transgression." If a man curses his friend and describes him as a donkey, and he believes that his friend resembles a donkey in his dullness, then this oath is not a false oath (yamin (definition) ghamus). However, if he believes otherwise, then it is a false oath (yamin ghamus) from which he must repent. An expiation (kaffarah (definition)) is due for it according to the Shafi'is, in contrast to the majority of scholars. As for one who doubts whether he has made false oaths or not, according to the majority of scholars, he owes nothing but repentance. However, according to the view that expiation is binding, he is not obligated to do anything merely based on doubt. Rather, he must offer expiation for the oaths that he is certain he made.