Is the persistence of viscosity or greasy residue from an impurity after washing it with water excused, and does excusing a slight impurity or one that is difficult to remove mean considering the item pure and that the impurity does not transfer?

The criterion for what is pardoned regarding impurity is explained in Fatwa no. 107496. The meaning of an impurity being pardoned is that its removal is not obligatory, and its persistence does not cause harm. The persistence of the stickiness of an impurity is evidence of the persistence of its substance. If it is among those impurities that are not pardoned, then it must be washed until it is removed. One should strive against obsessive thoughts (waswas) and free oneself from them, because succumbing to them leads to great evil.