What is the ruling regarding oil residue on underwear after using it to treat hemorrhoids, and is it considered impure (najis)?

If a person performs ablution (wudu') and then doubts whether something has exited from him, the fundamental principle is that the purity (tahara) remains, and ablution is not invalidated by doubt. Unless you are certain that the trace on your clothes originated from the anus, your purity is not invalidated, and the trace is not deemed impure. However, if you are certain that something has exited, even if it is a small amount, it invalidates ablution. Nevertheless, it is excused in terms of impurity, and there is no need to wash or change clothes.