[3] This passage from Utilitarianism seems to suggest that he was: In the case of abstinences indeed—of things which people forbear to do from moral considerations, though the consequences in the particular case might be beneficial—it would be unworthy of an intelligent agent not to be consciously aware that the action is of a class which, if practiced generally, would be generally injurious, and that this is the ground of the obligation to abstain from it.
But Mill also argues that it is sometimes right to violate general ethical rules: ... justice is a name for certain moral requirements, which, regarded collectively, stand higher in the scale of social utility, and are therefore of more paramount obligation, than any others; though particular cases may occur in which some other social duty is so important, as to overrule any one of the general maxims of justice.
Thus, to save a life, it may not only be allowable, but a duty, to steal, or take by force, the necessary food or medicine, or to kidnap, and compel to officiate, the only qualified medical practitioner.
A scenario (or thought experiment) used to clarify this problem (often attributed to Immanuel Kant) posits both The moral convention is that lying is wrong, so the strong rule utilitarian says you should reveal their location.
Weak rule utilitarianism (WRU) attempts to handle SRU counterexamples as legitimate exceptions.