I'm once in a similar situation (it's not about log data, but deal with ethics nonetheless)
Some of the answers mention about asking your superior to put the request in writing. That sound good and nice. But it is important to know your country culture and your boss intelligence as well.
For my case back then, my superior was intelligent enough to sense my plan of protecting myself while possibly putting him in danger(it was actually so obvious the moment you request/hints at getting something in writing), I don't want to get into the details, but things are changed and I'm removed very swiftly without me expecting it to be that fast. It was painful and depressing.
Also if such a culture (of covering up) is common in your country, your future employers knowing about it (I don't know about you, but over here, some companies job applications forms require you to state whether you have been fired or forced to resign before and state the reason) might view this as a negative trait rather than something positive.
In fact, there is this local news recently about a previously highly paid engineer who has been working as a rag and bone man for some years to support his family ever since he was 'laid off' after whistle blowing on his company. Even after the news is reported, he was still unable to get a proper real job (due to age, over qualified.etc) and that is a possible consequence one has to think about too.
Bottom line is, it's good to consider the ethical aspects of it, but do think of the bigger picture too. Can you afford to lose this job in your current circumstances? What is the current situation? is this really serious? Any backup plans?.etc
Also, consider getting proofs in less obvious ways such as instant messaging your superior, e.g. ('I kind of forgotten, so just to confirm, modify this xyz record to 5?') and the moment he reply to this instant message on skype.etc, you have a record already.
Still, if the conclusion is that you rather lose the job than to do such a terrible crime, then be the one who resign. At least you will feel better in the sense that it is you who ditch your superior, not the other way round.
Regardless of whether you will do it anot, try to plot your escape as soon as possible.